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Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2017	Mar. 09, 2018	Jun. 30, 2017
Document and Entity Information
Entity Registrant Name	Voyager Therapeutics, Inc.
Entity Central Index Key	0001640266
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	No
Entity Filer Category	Accelerated Filer
Entity Public Float			$ 94.4
Entity Common Stock, Shares Outstanding		32,206,786
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 31,530	$ 36,641
Marketable securities	137,522	137,777
Prepaid expenses and other current assets	2,738	4,368
Total Current Assets	171,790	178,786
Property, Plant and Equipment, Net	10,283	7,893
Deposits and other non-current assets	1,304	1,527
Marketable securities	1,100	1,360
Total Assets	184,477	189,566
Current liabilities:
Accounts Payable	1,020	550
Accrued expenses	11,497	6,488
Deferred revenue, current portion	3,380	6,764
Total current liabilities	15,897	13,802
Deferred rent, net of current portion	5,337	4,999
Deferred revenue, net of current portion	28,180	34,818
Other non-current liabilities	1,012	25
Total Liabilities	50,426	53,644
Stockholders’ equity:
Preferred stock, $0.001 par value: 5,000,000 shares authorized; no shares issued and outstanding at December 31, 2017 and 2016	0	0
Common stock, $0.001 par value: 120,000,000 shares authorized; 31,572,044 and 25,597,912 shares issued and outstanding at December 31, 2017 and 2016, respectively	32	26
Additional paid-in capital	295,019	225,963
Accumulated other comprehensive gain	(287)	(52)
Accumulated deficit	(160,713)	(90,015)
Total stockholders’ deficit	134,051	135,922
Total liabilities and stockholders’ equity	$ 184,477	$ 189,566

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Preferred Stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred Stock, Shares Authorized	5,000,000	5,000,000
Preferred Stock, Shares Issued	0	0
Preferred Stock, Shares Outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common Stock, Shares Authorized	120,000,000	120,000,000
Common Stock, Shares, Issued	31,572,044	25,597,912
Common Stock, Shares, Outstanding	31,572,044	25,597,912

Statement of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Collaboration revenue	$ 10,135	$ 14,220	$ 17,334
Operating expenses:
Research and development	62,260	42,249	27,679
General and administrative	19,738	13,270	9,909
Total operating expenses	81,998	55,519	37,588
Operating loss	(71,863)	(41,299)	(20,254)
Other income (expense), net:
Interest income	1,227	976	332
Other (expense) income, net	(62)	182	(9,750)
Total other income (expense), net	1,165	1,158	(9,418)
Loss before income taxes	(70,698)	(40,141)	(29,672)
Income tax provision		(52)
Net loss	(70,698)	(40,193)	(29,672)
Other comprehensive loss
Net unrealized (loss) gain on available-for-sale-securities, net of tax expense of $128 for the year ended December 31, 2016	(235)	199	(251)
Total other comprehensive (loss) income	(235)	199	(251)
Comprehensive loss	(70,933)	(39,994)	(29,923)
Reconciliation of net loss to net loss attributable to common stockholders:
Accretion of redeemable convertible preferred stock to redemption value			(7,373)
Accrued dividends on series A preferred stock			(1,245)
Net loss attributable to common stockholders	$ (70,698)	$ (40,193)	$ (38,290)
Net loss per share attributable to common stock, basic and diluted	$ 2.64	$ 1.59	$ 9.14
Weighted-average common shares outstanding, basic and diluted	26,803,711	25,302,414	4,191,210

Statement of Operations and Comprehensive Loss (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($)
Income Statement [Abstract]
Tax expense	$ 128

Consolidate Statements of Equity - USD ($) $ in Thousands	Redeemable Convertible Preferred Stock Series A	Redeemable Convertible Preferred Stock Series B	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit	Total
Stockholders' Equity, Beginning Balance at Dec. 31, 2014	$ 21,979		$ 1			$ (20,831)	$ (20,830)
Shares, Outstanding, Beginning Balance at Dec. 31, 2014	25,000,000		814,834
Issuance of common stock			$ 6	$ 72,948			72,954
Issuance of common stock (in shares)			5,750,000
Issuance of preferred stock, net of discount and issuance costs	$ 19,999	$ 84,780
Issuance of preferred stock, net of discount and issuance costs (in shares)	20,000,000	30,000,001
Reclassification of tranche rights upon issuance of Series A redeemable convertible preferred stock	$ 16,055
Conversion of redeemable convertible preferred stock to common stock	$ (60,182)	$ (90,005)	$ 18	144,675		5,494	150,187
Conversion of redeemable convertible preferred stock to common stock (common stock shares)			17,647,054
Conversion of redeemable convertible preferred stock to common stock (preferred stock shares)	(45,000,000)	(30,000,001)
Vesting of restricted common stock				22			22
Vesting of restricted common stock (in shares)			717,747
Exercises of vested stock options				10			10
Exercises of vested stock options (in shares)			1,344
Stock based compensation expense				4,027			4,027
Unrealized loss on available for-sale securities					$ (251)		(251)
Accretion of redeemable convertible preferred stock to redemption value	$ (2,149)	$ (5,225)		2,560		4,813	7,373
Net loss						(29,672)	(29,672)
Stockholders' Equity, Ending Balance at Dec. 31, 2015			$ 25	219,122	(251)	(49,822)	169,074
Shares, Outstanding, Ending Balance at Dec. 31, 2015			24,930,979
Vesting of restricted common stock			$ 1	17			18
Vesting of restricted common stock (in shares)			601,501
Exercises of vested stock options				514			514
Exercises of vested stock options (in shares)			65,432
Stock based compensation expense				6,310			6,310
Unrealized loss on available for-sale securities					199		199
Net loss						(40,193)	(40,193)
Stockholders' Equity, Ending Balance at Dec. 31, 2016			$ 26	225,963	(52)	(90,015)	135,922
Shares, Outstanding, Ending Balance at Dec. 31, 2016			25,597,912
Issuance of common stock			$ 5	57,989			57,994
Issuance of common stock (in shares)			5,175,000
Vesting of restricted common stock			$ 1	12			13
Vesting of restricted common stock (in shares)			573,803
Exercises of vested stock options				1,363			1,363
Exercises of vested stock options (in shares)			158,677
Stock based compensation expense				9,129			9,129
Unrealized loss on available for-sale securities					(235)		(235)
Issuance of common stock under ESPP				563			563
Issuance of common stock under ESPP (in shares)			66,652
Net loss						(70,698)	(70,698)
Stockholders' Equity, Ending Balance at Dec. 31, 2017			$ 32	$ 295,019	$ (287)	$ (160,713)	$ 134,051
Shares, Outstanding, Ending Balance at Dec. 31, 2017			31,572,044

Consolidated Statements of Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended	24 Months Ended
	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2017
Issuance costs	$ 200
Common Stock
Discounts and issuance costs	7,500	$ 7,500	$ 4,100
Redeemable Convertible Preferred Stock Series A
Issuance costs	1	32
Redeemable Convertible Preferred Stock Series B
Issuance costs	220
Discount	$ 5,000	$ 5,000

Statement of Cash Flows - USD ($) $ in Thousands	12 Months Ended
Statement of Cash Flows - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flow from operating activities
Net loss	$ (70,698)	$ (40,193)	$ (29,672)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	9,238	6,310	4,027
Depreciation	1,595	612	600
Amortization of premiums and discounts on marketable securities	(24)	696	452
Change in fair value of preferred stock tranche liability			9,750
In-kind research and development expenses	113	1,182	2,316
Other non-cash items	46	709	(277)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	1,630	(847)	(234)
Other non-current assets		7	14
Deferred revenue	(10,135)	(14,582)	52,666
Accounts payable	470	(62)	(942)
Accrued expenses	4,900	2,636	2,788
Other non-current liabilities	1,000		(189)
Lease incentive benefit	515	1,050
Net cash (used in) provided by operating activities	(61,350)	(42,482)	41,299
Cash flow from investing activities
Purchases of property and equipment	(3,985)	(5,029)	(1,030)
Change in restricted cash		(421)
Purchases of marketable securities	(147,296)	(112,350)	(220,399)
Proceeds from maturities or sales of marketable securities	147,600	165,100	26,660
Net cash (used in) provided by investing activities	(3,681)	47,300	(194,769)
Cash flow from financing activities
Proceeds from the issuance of stock net of discount and issuance costs	57,994		104,779
Proceeds from the exercise of stock options	1,363	514	72,965
Proceeds from the purchase of common stock under ESPP	563
Net cash provided by financing activities	59,920	514	177,744
Net (decrease) increase in cash and cash equivalents	(5,111)	5,332	24,274
Cash and cash equivalents, beginning of period	36,641	31,309	7,035
Cash and cash equivalents, end of period	$ 31,530	36,641	31,309
Supplemental Disclosure of Cash and non-cash Activities
Capital expenditures incurred but not yet paid		$ 242
Accretion of redeemable convertible preferred stock to redemption value			7,373
Conversion of redeemable convertible preferred stock to common stock			$ 150,187

Nature of business	12 Months Ended
Nature of business	Dec. 31, 2017
Nature of business
Nature of business	VOYAGER THERAPEUTICS INC. NOTES TO CONSOLIDATED FINANCIAL STATEMENTS 1. Nature of business The Company is a clinical-stage gene therapy company focused on developing life-changing treatments for patients suffering from severe neurological diseases. The Company is focused on neurological diseases where it believes an adeno-associated virus (“AAV”) gene therapy approach that either increases or decreases the production of a specific protein can slow or reduce the symptoms experienced by patients, and therefore have a clinically meaningful impact. The Company has built a product engine that it believes positions itself to be the leading company at the intersection of AAV gene therapy and severe neurological disease. The Company’s product engine enables it to engineer, optimize, manufacture and deliver its AAV-based gene therapies that have the potential to provide durable efficacy following a single administration. Additionally, the Company is working to identify novel AAV capsids, which are the outer viral protein shells that enclose the genetic material of the virus payload. The Company’s team of experts in the fields of AAV gene therapy and neuroscience first identifies and selects severe neurological diseases that are well-suited for treatment using AAV gene therapy. The Company then engineers and optimizes AAV vectors for delivery of the virus payload to the targeted tissue or cells. The Company’s manufacturing process employs an established system that it believes will enable production of high quality AAV vectors at commercial-scale. Finally, the Company leverages established routes of administration and advances in dosing techniques to optimize delivery of its AAV gene therapies to target cells that are critical to the disease of interest either directly to discrete regions of the brain or, more broadly, to the spinal cord region. The Company is devoting substantially all of its efforts to product research and development, market development, and raising capital. The Company is subject to risks common to companies in the biotechnology and gene therapy industry, including but not limited to, risks of failure of preclinical studies and clinical trials, the need to obtain marketing approval for its drug product candidates, the need to successfully commercialize and gain market acceptance of its drug product candidates, dependence on key personnel, protection of proprietary technology, compliance with government regulations, development by competitors of technological innovations, and ability to transition from pilot-scale manufacturing to large-scale production of products The Company has incurred annual net operating losses in every year since inception. As of December 31, 2017, the Company had an accumulated deficit of $160.7 million. The Company has not generated any product revenue and has financed its operations primarily through public offerings and private placements of its equity securities and funding from its collaboration with Sanofi Genzyme. There can be no assurance that the Company will be able to obtain additional debt or equity financing or generate product revenue or revenue from collaborative partners on terms acceptable to the Company, on a timely basis or at all. The failure of the Company to obtain sufficient funds on acceptable terms when needed could have a material adverse effect on the Company’s business, results of operations, and financial condition.

Summary of significant accounting policies

12 Months Ended

Dec. 31, 2017

Summary of significant accounting policies

Summary of significant accounting policies

2. Summary of significant accounting policies

The following is a summary of significant accounting policies followed in the preparation of these financial statements.

Basis of presentation

The accompanying consolidated financial statements include those of the Company and its subsidiary, Voyager Securities Corporation, after elimination of all intercompany accounts and transactions. The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”).

Public offerings

On October 29, 2015, in preparation for the Company’s IPO, the Company’s Board of Directors and stockholders approved a 1-for-4.25 reverse split of the Company’s common stock, which became effective on October 29, 2015. All share and per share amounts in the consolidated financial statements and notes thereto have been retroactively adjusted for all periods presented to give effect to this reverse split, including reclassifying an amount equal to the reduction in par value of common stock to additional paid-in capital.

On November 16, 2015, the Company completed the sale of 5,750,000 shares of its common stock in its initial public offering (the “IPO”), at a price to the public of $14.00 per share, resulting in net proceeds to the Company of $72.9 million after deducting underwriting discounts, commissions and offering expenses payable by the Company.

Upon the closing of the IPO, all of the Company’s outstanding redeemable convertible preferred stock automatically converted into shares of common stock, resulting in the issuance of an additional 17,647,054 shares of common stock. The significant increase in shares outstanding in November 2015 effected the year-over-year comparability of the Company’s net loss per share calculations.

On November 7, 2017, the Company completed the sale of 5,175,000 shares of its common stock in a public offering at a price to the public of $12.00 per share, resulting in net proceeds to the Company of $58.0 million after deducting underwriting discounts, commissions, and offering expenses payable by the Company.

Use of Estimates

The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. On an ongoing basis, the Company’s management evaluates its estimates, which include, but are not limited to, estimates related to revenue recognition, accrued expenses, valuation of the tranche rights, stock‑based compensation expense, income taxes and the fair value of common stock. The Company bases its estimates on historical experience and other market specific or other relevant assumptions that it believes to be reasonable under the circumstances. Actual results may differ from those estimates or assumptions.

Fair Value of Financial Instruments

ASC Topic 820, Fair Value Measurement (“ASC 820”), establishes a fair value hierarchy for instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company’s own assumptions (unobservable inputs). Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances.

ASC 820 identifies fair value as the exchange price, or exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As a basis for considering market participant assumptions in fair value measurements, ASC 820 establishes a three‑tier fair value hierarchy that distinguishes between the following:

Level 1—Quoted market prices in active markets for identical assets or liabilities.

Level 2—Inputs other than Level 1 inputs that are either directly or indirectly observable, such as quoted market prices, interest rates, and yield curves.

Level 3—Unobservable inputs developed using estimates of assumptions developed by the Company, which reflect those that a market participant would use.

To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.

The carrying amounts reflected in the balance sheets for cash and cash equivalents, prepaid expenses and other current assets, accounts payable and accrued expenses approximate their fair values, due to their short‑term nature.

Cash and Cash Equivalents

The Company considers all highly liquid investments purchased with original maturities of 90 days or less at acquisition to be cash equivalents. Cash and cash equivalents include cash held in banks and amounts held in money market funds.

Marketable Securities

The Company classifies marketable debt securities with a remaining maturity of greater than three months when purchased as available‑for‑sale. Marketable debt securities with a remaining maturity date greater than one year and marketable equity securities are classified as non‑current where the Company has the intent and ability to hold these securities for at least the next 12 months. During 2016, the Company invested in a supplier and received common stock and warrants to purchase common stock in that entity. The common stock is considered an available-for-sale marketable equity security and is included in non-current marketable securities, and the warrants are included in non-current assets.

All available for sale securities are carried at fair value with the unrealized gains and losses included in other comprehensive income (loss) as a component of stockholders’ equity until realized. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expense. Realized gains and losses are determined using the specific identification method and are included in other income (expense). If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other than temporary” and, if so, recognizes the loss through a charge to the Company’s statement of operations and comprehensive loss. No other than temporary losses have been recognized.

Cash, cash equivalents, and marketable securities as of December 31, 2017 and 2016 consist of the following:


	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
	(in thousands)
As of December 31, 2017
Money market funds included in cash and cash equivalents	$	30,469	$	—	$	—	$	30,469
Marketable securities:
U.S. Treasury notes		137,560		—		38		137,522
Equity securities		1,220		—		120		1,100
Total marketable securities	$	138,780	$	—	$	158	$	138,622
Total money market funds and marketable securities	$	169,249	$	—	$	158	$	169,091
As of December 31, 2016
Money market funds included in cash and cash equivalents	$	36,003	$	—	$	—	$	36,003
Marketable securities:
U.S. Treasury notes		130,237		2		66		130,173
U.S. Government agency bonds		7,604		—		—		7,604
Total debt securities	$	137,841	$	2	$	66	$	137,777
Equity securities		1,220		140		—		1,360
Total marketable securities	$	139,061	$	142	$	66	$	139,137
Total money market funds and marketable securities	$	175,064	$	142	$	66	$	175,140

All of the Company’s marketable debt securities at December 31, 2017 and 2016 have a contractual maturity of one year or less.

Restricted Cash

At December 31, 2017 and 2016, the Company maintained restricted cash totaling approximately $0.7 million held in the form of money market accounts as collateral for the Company’s facility lease obligation. The balance is included within deposits in other non‑current assets in the accompanying consolidated balance sheets.

Property and Equipment

Property and equipment consists of laboratory equipment, furniture and office equipment, and leasehold improvements and is stated at cost, less accumulated depreciation. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred; while costs of major additions and betterments are capitalized. Depreciation is calculated over the estimated useful lives of the assets using the straight‑line method.

Impairment of Long‑Lived Assets

The Company evaluates long‑lived assets for potential impairment when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparing the book values of the assets to the expected future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book values of the assets exceed their fair value. The Company has not recognized any impairment losses from inception through December 31, 2017.

Revenue Recognition

As of December 31, 2017, all of the Company’s revenue is generated exclusively from its collaboration agreement with Sanofi Genzyme Corporation, a Sanofi company (“Sanofi Genzyme”).

The Company recognizes revenue in accordance with ASC Topic 605, Revenue Recognition (“ASC 605”). Accordingly, revenue is recognized for each unit of accounting when all of the following criteria are met:

Persuasive evidence of an arrangement exists;

Delivery has occurred or services have been rendered;

The seller’s price to the buyer is fixed or determinable; and

Collectability is reasonably assured.

Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue in the Company’s balance sheet. Amounts expected to be recognized as revenue within 12 months following the balance sheet date are classified as deferred revenue, current portion. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are included in noncurrent liabilities and classified as deferred revenue, net of current portion.

The Company analyzes the multiple element arrangements based on the guidance in ASC Topic 605‑25, Revenue Recognition—Multiple Element Arrangements (“ASC 605‑25”). Pursuant to the guidance in ASC 605‑25, the Company evaluates multiple element arrangements to determine (1) the deliverables included in the arrangement and (2) whether the individual deliverables represent separate units of accounting or whether they must be accounted for as a combined unit of accounting. This evaluation involves subjective determinations and requires management to make judgments about the individual deliverables and whether such deliverables are separate from other aspects of the contractual relationship. Deliverables are considered separate units of accounting provided that: (i) the delivered item(s) has value to the customer on a standalone basis and (ii) if the arrangement includes a general right of return relative to the delivered item(s), delivery or performance of the undelivered item(s) is considered probable and substantially within control of the Company. In assessing whether an item has standalone value, the Company considers factors such as the research, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can use the other deliverable(s) for their intended purpose without the receipt of the remaining element(s), whether the value of the deliverable is dependent on the undelivered item(s) and whether there are other vendors that can provide the undelivered element(s). The Company’s collaboration agreement does not contain a general right of return relative to any delivered items.

Arrangement consideration that is fixed or determinable is allocated among the separate units of accounting using the relative selling price method. Then the applicable revenue recognition criteria in ASC 605 are applied to each of the separate units of accounting in determining the appropriate period and pattern of recognition. The Company determines the selling price of a unit of accounting following the hierarchy of evidence prescribed by ASC 605‑25. Accordingly, the Company determines the estimated selling price for units of accounting within each arrangement using vendor specific objective evidence (“VSOE”) of selling price, if available, third-party evidence (“TPE”) of selling price if VSOE is not available, or best estimate of selling price (“BESP”) if neither VSOE or TPE is available. The Company has only used BESP to estimate the selling price, since it has not had VSOE or TPE of selling price of any units of accounting to date. Determining BESP for a unit of accounting requires significant judgment. In developing the BESP for a unit of accounting, the Company considers applicable market conditions and relevant entity specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. The Company validates BESP for units of accounting by evaluating whether changes in the key assumptions used to determine the BESP will have a significant effect on the allocation of arrangement consideration between multiple units of accounting.

Options are considered substantive if, at the inception of the arrangement, the Company is at risk as to whether the collaboration partner will choose to exercise the option. Factors that the Company considers in evaluating whether an option is substantive include the cost to exercise the option, the overall objective of the arrangement, the benefit the collaborator might obtain from the arrangement without exercising the option and the likelihood the option will be exercised. When an option is considered substantive, the Company does not consider the option or item underlying the option to be a deliverable at the inception of the arrangement and the associated option fees are not included in allocable consideration, assuming the option is not priced at a significant and incremental discount. Conversely, when an option is not considered substantive, the Company would consider the option, including other deliverables contingent upon the exercise of the option, to be a deliverable at the inception of the arrangement and a corresponding amount would be included in allocable arrangement consideration. In addition, if the price of the option includes a significant incremental discount, the option would be included as a deliverable at the inception of the arrangement.

The Company recognizes arrangement consideration allocated to each unit of accounting when all of the revenue recognition criteria in ASC 605 are satisfied for that particular unit of accounting. The Company will recognize revenue associated with license options upon exercise of the option, if the underlying license has standalone value from the other deliverables to be provided subsequent to delivery of the license. If the license does not have standalone value, the amounts allocated to the license option will be combined with the related undelivered items as a single unit of accounting. The amounts allocated to the license option in the Sanofi Genzyme agreement will be deferred until the option is exercised. The revenue recognition upon option exercise will be determined based on whether the license has standalone value from the remaining deliverables under the arrangement at the time of exercise.

The Company recognizes the amounts associated with research and development services, alliance joint steering committees and development advisory committees ratably over the associated period of performance. If there is no discernible pattern of performance and/or objectively measurable performance measures do not exist, then the Company recognizes revenue under the arrangement on a straight‑line basis over the period the Company is expected to complete its performance obligations. Conversely, if the pattern of performance in which the service is provided to the customer can be determined and objectively measurable performance exist, then the Company recognizes revenue under the arrangement using the proportional performance method. Revenue recognized is limited to the lesser of the cumulative amount of payments received or the cumulative revenue earned determined using the straight line method or proportional performance, as applicable, as of the period end date.

At the inception of an arrangement that includes milestone payments, the Company evaluates whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone. This evaluation includes an assessment of whether: (i) the consideration is commensurate with either the Company’s performance to achieve the milestone or the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from the Company’s performance to achieve the milestone, (ii) the consideration relates solely to past performance and (iii) the consideration is reasonable relative to all the deliverables and payment terms within the arrangement. The Company evaluates factors such as clinical, regulatory, commercial and other risks that must be overcome to achieve the respective milestone and the level of effort and investment required to achieve the respective milestone in making this assessment. There is considerable judgment involved in determining whether a milestone satisfies all the criteria required to conclude that a milestone is substantive. In accordance with ASC Topic 605‑28, Revenue Recognition—Milestone Method (“ASC 605‑28”) clinical and regulatory milestones that are considered substantive, will be recognized as revenue in their entirety upon successful accomplishment of the milestone, assuming all other revenue recognition criteria are met. Milestones that are not considered substantive would be recognized as revenue over the remaining period of performance, assuming all other revenue recognition criteria are met. Revenue from commercial milestone payments will be accounted for as royalties and recorded as revenue upon achievement of the milestone, assuming all other revenue recognition criteria are met.

The Company will recognize royalty revenue in the period of sale of the related product(s), based on the underlying contract terms, provided that the reported sales are reliably measurable and the Company has no remaining performance obligations, assuming all other revenue recognition criteria are met.

The Company also considers the impact of potential future payments it makes in its role as a vendor to its customers or collaboration partners and evaluates if these potential future payments could be reductions of revenue from that customer. If the potential future payments to the customer are (i) a separately identifiable benefit and (ii) the fair value of the identifiable benefit can be reasonably estimated, then the payments are accounted for separately from the revenue received from the customer. If however, both of these criteria are not satisfied, then the payments are treated as a reduction of revenue.

Research and Development

Research and development costs are charged to expense as incurred in performing research and development activities. The costs include employee compensation costs, external research, consultant costs, sponsored research, in‑kind services provided under the Sanofi Genzyme agreement, license fees, process development and facilities costs. Facilities costs primarily include the allocation of rent, utilities and depreciation.

Research Contract Costs and Accruals

The Company has entered into various research and development contracts with research institutions and other companies. These agreements are generally cancelable. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.

Patent Costs

The Company expenses patent application and related legal costs as incurred and classifies such costs as general and administrative expenses in the accompanying statements of operations.

Stock‑Based Compensation Expense

The Company accounts for its stock‑based compensation awards in accordance with ASC Topic 718 Compensation—Stock Compensation (“ASC 718”). ASC 718 requires all stock‑based payments to employees and directors, including grants of restricted stock and stock options, to be recognized as expense in the statements of operations based on their grant date fair values. Grants of restricted stock and stock options to other service providers, referred to as non‑employees, are required to be recognized as expense in the statements of operations based on their vesting date fair values. The Company estimates the fair value of options granted using the Black‑Scholes option pricing model. The Company uses the fair value of its common stock to determine the fair value of restricted stock awards.

The Black‑Scholes option pricing model requires inputs based on certain subjective assumptions, including (a) the expected stock price volatility, (b) the calculation of expected term of the award, (c) the risk‑free interest rate and (d) expected dividends. Due to a lack of company-specific historical and implied volatility data, the Company bases the estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded, blended with the most recent period of historic volatility of its common stock. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The computation of expected volatility is based on the historical volatility of a representative group of companies with similar characteristics to the Company, including stage of product development and life science industry focus. The Company uses the simplified method as prescribed by the SEC Staff Accounting Bulletin No. 107, Share‑Based Payment, to calculate the expected term for stock options granted to employees as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term. For stock options granted to non‑employees, the Company utilizes the contractual term of the arrangement as the basis for the expected term assumption. The risk‑free interest rate is based on a treasury instrument whose term is consistent with the expected term of the stock options. The expected dividend yield is assumed to be zero as the Company has never paid dividends and has no current plans to pay any dividends on its common stock.

The Company expenses the fair value of its stock‑based compensation awards to employees on a straight‑line basis over the associated service period, which is generally the period in which the related services are received. Stock‑based compensation awards to non‑employees are adjusted through stock‑based compensation expense at each reporting period end to reflect the current fair value of such awards and are expensed on a straight‑line basis.

The Company records the expense for stock‑based compensation awards subject to performance conditions over the remaining service period when management determines that achievement of the performance condition is probable. Management evaluates when the achievement of a performance condition is probable based on the expected satisfaction of the performance conditions as of the reporting date.

Income Taxes

Income taxes are recorded in accordance with ASC Topic 740, Income Taxes (“ASC 740”), which provides for deferred taxes using an asset and liability approach. Under this method, deferred tax assets and liabilities are determined based on the difference between the financial reporting and the tax reporting basis of assets and liabilities and are measured using the enacted tax rates and laws that are expected to be in effect when the differences are expected to reverse. The Company provides a valuation allowance against net deferred tax assets unless, based upon the weight of available evidence, it is more likely than not that the deferred tax assets will be realized.

The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. As of December 31, 2017, the Company does not have any significant uncertain tax positions.

Comprehensive Loss

Comprehensive loss is comprised of net loss and other comprehensive income or loss. Other comprehensive gain or loss consists of unrealized gains or losses on marketable securities.

Net Loss Per Share Attributable to Common Stockholders

Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted‑average number of shares of common stock outstanding during the period, without consideration for potentially dilutive securities. Diluted net loss per share attributable to common stockholders is computed by dividing the net loss attributable to common stockholders by the weighted‑average number of shares of common stock and potentially dilutive securities outstanding for the period determined using the treasury‑stock and if‑converted methods.

For purposes of the diluted net loss per share attributable to common stockholders calculation, redeemable convertible preferred stock, unvested restricted common stock, and outstanding stock options are considered to be potentially dilutive securities, but are excluded from the calculation of diluted net loss per share attributable to common stockholders because their effect would be anti‑dilutive and therefore, basic and diluted net loss per share attributable to common stockholders were the same for all periods presented.

The following table sets forth the outstanding potentially dilutive securities that have been excluded in the calculation of diluted net loss per share attributable to common stockholders because to do so would be anti‑dilutive:


	As of December 31,
	2017	2016	2015
Unvested restricted common stock	557,979	1,167,984	1,818,261
Shares reserved for issuance under stock plans	5,375,431	4,226,265	2,905,458
Total	5,933,410	5,394,249	4,723,719

All of the Company’s outstanding convertible preferred stock automatically converted into shares of common stock as of November 16, 2015, resulting in the issuance by the Company of an additional 17,647,054 shares of common stock.

Concentrations of Credit Risk and Off‑Balance Sheet Risk

The Company has no financial instruments with off‑balance sheet risk such as foreign exchange contracts, option contracts or other foreign hedging arrangements. Financial instruments that potentially subject the Company to a concentration of credit risk are cash and cash equivalents. The Company’s cash is held in accounts at a financial institution that may exceed federally insured limits. The Company has not experienced any credit losses in such accounts and does not believe it is exposed to any significant credit risk on these funds.

Concentration of Suppliers

The Company is dependent on a third‑party manufacturer to supply certain products for research and development activities in its programs. In particular, the Company relies on a sole manufacturer to supply it with specific vectors related to the Company’s research and development programs.

Segment Information

Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker in deciding how to allocate resources and assess performance. The Company and the Company’s chief operating decision maker, the Company’s Chief Executive Officer, views the Company’s operations and manages its business as a single operating segment, which is the business of developing and commercializing gene therapies.

Recent Accounting Pronouncements

In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”). Subsequently, the FASB also issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606), which adjusted the effective date of ASU 2014-09; ASU No. 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net), which amends the principal-versus-agent implementation guidance and illustrations in ASU 2014-09; ASU No. 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing, which clarifies identifying performance obligation and licensing implementation guidance and illustrations in ASU 2014-09; and ASU No. 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients, which addresses implementation issues and is intended to reduce the cost and complexity of applying the new revenue standard in ASU 2014-09 (collectively, the “Revenue ASUs”).

The Revenue ASUs provide an accounting standard for a single comprehensive model for use in accounting for revenue arising from contracts with customers and supersedes most current revenue recognition guidance. The accounting standard is effective for interim and annual periods beginning after December 15, 2017, with an option to early adopt for interim and annual periods beginning after December 15, 2016. The guidance permits two methods of adoption: retrospectively to each prior reporting period presented (the full retrospective method), or retrospectively with the cumulative effect of initially applying the guidance recognized at the date of initial application (the modified retrospective method).

As of December 31, 2017, the Company had one revenue generating arrangement, which was the Sanofi Genzyme Collaboration, as defined below. The Company has elected to adopt the Revenue ASUs effective as of January 1, 2018, under the modified retrospective method. The Company has substantially completed its assessment of the effect that this new standard will have on its consolidated financial statements. The most significant effect of the adoption is expected to be the method of revenue recognition for services performed over time. Under the previous accounting standards, revenue was recognized over the estimated period of performance while revenue will be recognized based on a proportional performance model under the Revenue ASUs. This change in accounting policy, along with certain other factors, is expected to result in an adjustment of approximately $18.0 million to $23.0 million, representing an increase of deferred revenue with an offset to accumulated deficit on January 1, 2018 to reflect the cumulative effect of the accounting changes made upon the adoption of the standard. In addition, the Company expects that the changes in accounting for contingent milestone payments will have a significant effect on the future accounting treatment for the arrangement. The previous accounting guidance contained specific guidance related to the accounting for milestone payments, including, if certain criteria were met, the ability to recognize all consideration related to the milestone once that milestone was achieved. The Revenue ASUs do not contain guidance specific to milestone payments, thereby requiring potential milestone payments to be considered in accordance with the overall revenue recognition model. As a result, revenue from contingent milestone payments may be recognized earlier under the Revenue ASUs than under ASC 605, based on an assessment of the probability of achievement of the milestone and the likelihood of a significant reversal of such milestone revenue at each reporting date. This assessment may result in recognizing milestone revenue before the milestone event has been achieved. As of the adoption date, no consideration from contingent milestones was included in the arrangement consideration. The finalization of the Company’s assessment may result in significant changes to its estimates that may materially impact its preliminary estimate of the cumulative effect. The Company has implemented appropriate changes to its internal controls to support revenue recognition and additional revenue-related disclosures under the new standards.

In October 2016, the FASB issued ASU No. 2016-16, Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory (“ASU 2016-16”), which removes the prohibition in ASC 740 against the immediate recognition of the current and deferred income tax effects of intra-entity transfers of assets other than inventory. ASU 2016-16 is effective for the Company for annual periods beginning after December 15, 2017, and interim periods within those annual periods. Early adoption is permitted as of the beginning of a fiscal year for which neither the annual nor the interim (if applicable) financial statements have been issued. The Company is currently in the process of evaluating the impact that the adoption of this guidance will have on its consolidated financial statements and related disclosures.

In February 2016, the FASB issued ASU No. 2016-02, a comprehensive new lease accounting standard, which provides revised guidance on accounting for lease arrangements by both lessors and lessees and requires lessees to recognize a lease liability and a right-of-use asset for most leases. The new guidance is effective for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years, with early adoption permitted. The new standard must be applied using a modified retrospective transition approach which requires application of the new guidance for all periods presented. The Company is currently in the process of evaluating the impact that the adoption of this guidance will have on its consolidated financial statements and related disclosures.

Fair Value Measurements

12 Months Ended

Dec. 31, 2017

Fair Value Measurements

Fair value of financial measurements

3. Fair value measurements

Assets and liabilities measured at fair value on a recurring basis as of December 31, 2017 and 2016 are as follows:


			Quoted Prices		Significant
			in Active		Other		Significant
			Markets for		Observable		Unobservable
			Identical Assets		Inputs		Inputs
Assets	Total		(Level 1)		(Level 2)		(Level 3)
	(in thousands)
December 31, 2017
Money market funds included in cash and cash equivalents	$	30,469	$	30,469	$	—	$	—
Marketable securities:
U.S. Treasury notes		137,522		137,522		—		—
Equity securities		1,100		1,100		—		—
Total marketable securities	$	138,622	$	138,622	$	—	$	—
Warrants to purchase equity securities		569		—		569		—
Total	$	169,660	$	169,091	$	569	$	—
December 31, 2016
Money market funds included in cash and cash equivalents	$	36,003	$	36,003	$	—	$	—
Marketable securities:
U.S. Treasury notes		130,173		130,173		—		—
U.S. Government agency securities		7,604		—		7,604		—
Equity securities		1,360		1,360		—		—
Total marketable securities	$	139,137	$	131,533	$	7,604	$	—
Warrants to purchase equity securities		792		—		792		—
Total	$	175,932	$	167,536	$	8,396	$	—

The Company measures the fair value of money market funds, U.S. Treasuries and equity securities based on quoted prices in active markets for identical securities. The Level 2 debt securities include U.S. Government agency securities that are valued either based on recent trades of securities in inactive markets or based on quoted market prices of similar instruments and other significant inputs derived from or corroborated by observable market data. The Level 2 equity securities include warrants used to purchase equity securities that are valued using the Black-Scholes model. The Black-Scholes option pricing model requires inputs based on certain subjective assumptions, including (a) the expected stock price volatility, (b) the calculation of expected term of the awards, (c) the risk-free interest rate, and (d) expected dividends. The assumptions utilized to value the warrants to purchase equity securities as of December 31, 2017 and 2016 are as follows:


	As of December 31,
	2017		2016
Risk-free interest rate	2.0	%	1.8	%
Expected dividend yield	—	%	—	%
Expected term (in years)	3.7		4.7
Expected volatility	103.5	%	97.5	%

The expected volatility is based on the historic volatility for the equity securities underlying the warrants and is calculated based on a period of time commensurate with the expected term assumption. The expected term is based on the remaining contractual life of the warrants on each measurement date. The risk-free interest rate is based on a treasury instrument whose term is consistent with the expected term of the warrants. The expected dividend yield is assumed to be zero as the entity that issued the warrants has never paid and has not indicated any intention to pay dividends.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Prepaid expenses and other current assets

12 Months Ended

Dec. 31, 2017

Prepaid expense and other current assets

Prepaid expenses and other current assets

4. Prepaid expenses and other current assets

Prepaid expenses and other current assets consist of the following:


	As of December 31,
	2017		2016
	(in thousands)
Prepaid research and development contracts	$	1,330	$	1,094
Other current assets		766		541
Prepaid insurance		520		430
Accrued interest receivable		122		339
Tenant improvement receivable		—		1,964
Total	$	2,738	$	4,368

Property and equipment, net

12 Months Ended

Dec. 31, 2017

Property and equipment, net

Property and equipment, net

5. Property and equipment, net

Property and equipment, net consists of the following:


	As of December 31,
	2017		2016
	(in thousands)
Leasehold improvements	$	6,421	$	1,341
Laboratory equipment		5,262		3,306
Furniture and office equipment		1,565		526
Other		25		242
Construction in progress		—		3,873
Total property and equipment		13,273		9,288
Less: accumulated depreciation		(2,990)		(1,395)
Property and equipment, net	$	10,283	$	7,893

The Company recorded $1.6 million, $0.6 million, and $0.6 million in depreciation expense during the years ended December 31, 2017, 2016, and 2015, respectively. Construction-in-progress as of December 31, 2016 includes $3.0 million related to costs which were reimbursable by the landlord. Refer to Note 7 “Commitments and contingencies” for further details.

Accrued expenses

12 Months Ended

Dec. 31, 2017

Accrued expenses

Accrued expenses

6. Accrued expenses

Accrued expenses consist of the following:


	As of December 31,
	2017		2016
	(in thousands)
Research and development costs	$	5,780	$	2,384
Employee compensation costs		3,383		2,399
Professional services		1,762		698
Accrued goods and services		388		842
Patent costs		120		89
Other		64		76
Total	$	11,497	$	6,488

Commitments and contingencies

12 Months Ended

Dec. 31, 2017

Commitments and Contingencies

Commitments and contingencies

7. Commitments and contingencies

Operating Leases

During March 2014, the Company entered into an agreement to lease the 75 Sidney Street facility under a non‑cancelable operating lease that would expire December 15, 2019. The lease includes two renewal options, each for five-year terms and at fair market value upon exercise. The lease contains escalating rent clauses which require higher rent payments in future years. The Company expenses rent on a straight‑line basis over the term of the lease, including any rent‑free periods.

In January 2016, the Company executed an amendment to extend the 75 Sidney Street lease and executed an agreement to lease the 64 Sidney Street facility until December 31, 2024. The additional facility includes laboratory and office space, and was ready for occupancy in early 2017.

In February 2018, the Company executed an amendment to lease an additional approximately 11,000 square feet at 75 Sidney Street that is intended to support its continued growth. The additional facility includes laboratory and office space, and will be ready for occupancy in the first half of 2018.

The table below includes estimated payments related to the amended 75 Sidney Street lease and the lease for 64 Sidney Street as of December 31, 2017 and does not include the effect of the amendment executed in February 2018.

The Company has received leasehold improvement incentives from the landlord totaling $1.3 million and $3.5 million for 75 Sidney Street and 64 Sidney Street, respectively, as of December 31, 2017. The Company recorded these incentives as a component of deferred rent and is amortizing these incentives as a reduction of rent expense over the life of the lease. The leasehold improvements have been recorded as fixed assets. The Company is entitled to receive approximately $0.3 million of leasehold improvements for the space related to the amendment of the lease executed in February 2018.

Rent expense of approximately $2.9 million, $2.0 million, and $0.9 million was incurred during the years ended December 31, 2017, 2016, and 2015, respectively.

Future annual minimum lease payments at December 31, 2017 are as follows:


	Total Minimum
	Lease Payments
	(in thousands)
2018		3,290
2019		3,382
2020		3,762
2021		3,868
2022		3,977
2023+		8,295
	$	26,574

Significant Agreements

Sanofi Genzyme Collaboration Agreement

Summary of Agreement

In February 2015, the Company entered into an agreement with Sanofi Genzyme (the “Sanofi Genzyme Collaboration Agreement”) which included a non‑refundable upfront payment of $65.0 million. In addition, contemporaneous with entering into the Sanofi Genzyme Collaboration Agreement, Sanofi Genzyme entered into a Series B Stock Purchase Agreement, under which Sanofi Genzyme purchased 10,000,000 shares of Series B Preferred Stock for $30.0 million. The fair value of the Series B Preferred Stock at the time of issuance was approximately $25.0 million. The $5.0 million premium over the fair value is accounted for as additional consideration under the Sanofi Genzyme Collaboration Agreement.

Under the Sanofi Genzyme Collaboration Agreement, the Company granted Sanofi Genzyme an exclusive option to license, develop and commercialize (i) ex‑U.S. rights to the following programs, which are referred to as Split Territory Programs; VY‑AADC (“Parkinson’s Program”), VY‑FXN01 (“Friedreich’s Ataxia Program”), a future program to be designated by Sanofi Genzyme (“Future Program), and VY‑HTT01 (“Huntington’s Program”), with an incremental option to co‑commercialize VY‑HTT01 in the United States and (ii) worldwide rights to VY‑SMN101 (“Spinal Muscular Atrophy Program”). Sanofi Genzyme’s option for the Split Territory Programs and the Spinal Muscular Atrophy Program is triggered following the completion of the first proof‑of‑principle human clinical study (“POP Study”), on a program by program basis.

Prior to any option exercise by Sanofi Genzyme, the Company will collaborate with Sanofi Genzyme in the development of products under each Split Territory Program and the Spinal Muscular Atrophy Program pursuant to a written development plan and under the guidance of an Alliance Joint Steering Committee (“AJSC”), comprised of an equal number of employees from the Company and Sanofi Genzyme.

The Company is required to use commercially reasonable efforts to develop products under each Split Territory Program and the Spinal Muscular Atrophy Program through the completion of the applicable POP Study. During the development of these joint programs, the activities are guided by a Development Advisory Committee (“DAC”). The DAC may elect to utilize certain Sanofi Genzyme technology relating to the Parkinson’s Program, the Huntington’s Program or generally with the manufacture of Split Territory Program products.

The Company is solely responsible for all costs incurred in connection with the development of the Split Territory Programs and the Spinal Muscular Atrophy Program products prior to the exercise of an option by Sanofi Genzyme with the exception of the following: (i) at the Company’s request and upon mutual agreement, Sanofi Genzyme will provide “in‑kind” services valued at up to $5.0 million and (ii) Sanofi Genzyme shall be responsible for the costs and expenses of activities under the Huntington’s Program development plan to the extent such activities are covered by financial support Sanofi Genzyme is entitled to receive from a patient advocacy group, collectively Sanofi Genzyme “in‑kind” and other funding.

Other than the Parkinson’s Program (for which a POP Study has already been completed), if the Company does not initiate a POP Study for a given Split Territory Program by December 31, 2026 (or for the Future Program by the tenth anniversary of the date the Future Program is nominated by Sanofi Genzyme), and Sanofi Genzyme has not terminated the Sanofi Genzyme Collaboration Agreement with respect to the collaboration program, then Sanofi Genzyme shall be entitled, as its sole and exclusive remedy, to a credit of $10.0 million for each such program against other milestone or royalty payments payable by Sanofi Genzyme under the Sanofi Genzyme Collaboration Agreement. However, if the POP Study is not initiated due to a regulatory delay or a force majeure event, such time period shall be extended for so long as such delay continues.

With the exception of the Parkinson’s Program, Sanofi Genzyme is required to pay an option exercise payment of $20.0 million or $30.0 million for each Split Territory Program, as well as the Spinal Muscular Atrophy Program.

Upon Sanofi Genzyme’s exercise of its option to license a given product in a Split Territory Program (“Split Territory Licensed Product”), the Company will have sole responsibility for the development of such Split Territory Licensed Product in the United States and Sanofi Genzyme shall have sole responsibility for development of such Split Territory Licensed Product in the rest of the world. The Company and Sanofi Genzyme will have shared responsibility for execution of ongoing development of such Split Territory Licensed Product that is not specific to either territory, including costs associated therewith. The Company is responsible for all commercialization activities relating to Split Territory Licensed Products in the United States, including all of the associated costs. Sanofi Genzyme is responsible for all commercialization activities relating to the Split Territory Licensed Products in the rest of the world, including all of the associated costs. If Sanofi Genzyme exercised its co‑commercialization rights, Sanofi Genzyme will be the lead party responsible for all commercialization activities related to Huntington’s Licensed Product in the United States.

Upon exercise of the option, Sanofi Genzyme shall have the sole right to develop the Spinal Muscular Atrophy Product worldwide. Sanofi Genzyme shall be responsible for all of the development costs that occur after the option exercise date for the Spinal Muscular Atrophy Program. Sanofi Genzyme is also responsible for commercialization activities relating to the Spinal Muscular Atrophy Product worldwide.

Sanofi Genzyme is required to pay the Company for specified regulatory and commercial milestones, if achieved, up to $540.0 million across all programs. The Company is no longer entitled to receive a total of $105.0 million related to regulatory and commercial milestone payments for VY-AADC as a result of Sanofi Genzyme’s decision to not exercise its option for the Parkinson’s Program (the “PD Opt-Out”). The regulatory approval milestones are payable upon either regulatory approval in the United States or regulatory and reimbursement approval in the European Union and range from $40.0 million to $50.0 million per milestone, with an aggregate total of $220.0 million, after accounting for the PD Opt-Out. The commercial milestones are payable upon achievement of specified annual net sales in each program and range from $50.0 million to $100.0 million per milestone, with an aggregate total of $320.0 million, after accounting for the PD Opt-Out.

In addition, to the extent any Split Territory Licensed Products or the Spinal Muscular Atrophy Licensed Product are commercialized, the Company is entitled to tiered royalty payments ranging from the mid‑single digits to mid‑teens based on a percentage of net sales by Sanofi Genzyme. Sanofi Genzyme is entitled to receive tiered royalty payments related to sales of Split Territory Licensed Product ranging from the low‑single digits to mid‑single digits based on a percentage of net sales by the Company depending on whether the Company uses Sanofi Genzyme technology in the Split Territory Licensed Product. If Sanofi Genzyme elects to co‑commercialize VY‑HTT01 in the United States, the Company and Sanofi Genzyme will share in any profits or losses from VY‑HTT01 product sales.

The Sanofi Genzyme Collaboration Agreement will continue in effect until the later of (i) the expiration of the last to expire of the option rights and (ii) the expiration of all payment obligations unless sooner terminated by the Company or Sanofi Genzyme. The Company and Sanofi Genzyme have customary termination rights including the right to terminate for an uncured material breach of the agreement committed by the other party and Sanofi Genzyme has the right to terminate for convenience.

Accounting Analysis

The Sanofi Genzyme Collaboration Agreement includes the following deliverables: (i) research and development services for each of the Split Territory License Programs and the Spinal Muscular Atrophy Program, (ii) participation in the AJSC, (iii) participation in the DAC and (iv) the option to obtain a development and commercial license in the Parkinson’s Program and related deliverables. The Company has determined that the option to obtain a development and commercial license in the Parkinson’s program is not a substantive option for accounting purposes, primarily because there is no additional option exercise payment payable by Sanofi Genzyme at the time the option is exercised. Therefore, the option to obtain a license and other obligations of the Company that are contingent upon exercise of the option are considered deliverables at the inception of the arrangement. The options in the other Split Territory Programs and the Spinal Muscular Atrophy Program are considered substantive as there is substantial option exercise payments payable by Sanofi Genzyme upon exercise. In addition, as a result of the uncertainties related to the discovery, research, development and commercialization activities, the Company is at risk with regard to whether Sanofi Genzyme will exercise the options. Moreover, the substantive options are not priced at a significant incremental discount. Accordingly, the substantive options are not considered deliverables at the inception of the arrangement and the associated option exercise payments are not included in allocable arrangement consideration. The Company has also determined that any obligations which are contingent upon the exercise of a substantive option are not considered deliverables at the outset of the arrangement, as these deliverables are contingent upon the exercise of the options. In addition, any option exercise payments associated with the substantive options are not included in the allocable arrangement consideration.

The Company has concluded that each of the deliverables identified at the inception of the arrangement has standalone value from the other undelivered elements. Additionally, the Sanofi Genzyme Collaboration Agreement does not include return rights related to the initial collaboration term. Accordingly, each deliverable qualifies as a separate unit of accounting.

The Company has identified $79.3 million of allocable arrangement consideration consisting of the $65.0 million upfront fee, the $5.0 million premium paid in excess of fair value of the Series B Preferred Stock and $9.3 million of Sanofi Genzyme “in‑kind” and other funding.

The Company has allocated the allocable arrangement consideration based on the relative selling price of each unit of accounting. For all units of accounting, the Company determined the selling price using the BESP. The Company determined the BESP for the service related deliverable for the research and development activities based on internal estimates of the costs to perform the services, including expected internal expenses and expenses with third parties for services and supplies, marked up to include a reasonable profit margin and adjusted for the scope of the potential license. Significant inputs used to determine the total expense of the research and development activities include, the length of time required and the number and costs of various studies that will be performed to complete the POP Study. The BESP for the AJSC and DAC have been estimated based on the costs incurred to participate in the committees, marked up to include a reasonable profit margin. The BESP for the license option was determined based on the estimated value of the license and related deliverables adjusted for the estimated probability that the option would be exercised by Sanofi Genzyme.

Based on the relative selling price allocation, the allocable arrangement consideration was allocated as follows:


Unit of Accounting	Amount
	(in thousands)
Research and Development Services for:
Huntington’s Program	$	15,662
Parkinson’s Program		6,648
Friedreich’s Ataxia Program		16,315
Spinal Muscular Atrophy Program		32,050
Future Program		2,464
Committee Obligations:
AJSC		147
DAC		227
License Option and related deliverables		5,743
Total	$	79,256

The Company recognizes the amounts associated with research and development services on a straight-line basis over the period of service as there is no discernable pattern or objective measure of performance for the services. Similarly, the Company recognizes the amount associated with the committee obligations on a straight-line basis over the period of service consistent with the expected pattern of performance. The amount allocated to the license option was deferred until October 2017, when Sanofi Genzyme decided that it would not exercise its option to the Parkinson’s Program (the “PD Option”).

In October 2017, Sanofi Genzyme decided not to exercise the PD Option. Therefore, in the year ended December 31, 2017, the Company recognized revenue of $5.5 million of consideration which had been allocated to the PD Option. In addition, revenue recognized during the years ended December 31, 2017 and 2016 include amounts recognized related to consideration allocated to research and development services for various programs under the Sanofi Genzyme Collaboration Agreement. During 2017 the Company reassessed the estimated period of performance for each of the units of accounting and determined that the estimated period would be extended for two units of accounting. During 2016 the Company deprioritized the development of VY-SMN101 for the treatment of Spinal Muscular Atrophy. As a result, the Company ceased recognizing the revenue allocated to this program. These adjustments were made on a prospective basis and resulted in decreases in revenue recognized by $2.1 million and $9.5 million, respectively, for the year ended December 31, 2017.

The Company has evaluated all of the milestones that may be received in connection with the Split Territory Licensed Product and the Spinal Muscular Atrophy Program Licensed Product. In evaluating if a milestone is substantive, the Company assesses whether: (i) the consideration is commensurate with either the Company’s performance to achieve the milestone or the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from the Company performance to achieve the milestone, (ii) the consideration relates solely to past performance and (iii) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. All regulatory milestones are considered substantive on the basis of the contingent nature of the milestone, specifically reviewing factors such as the scientific, clinical, regulatory, and other risks that must be overcome to achieve the milestone as well as the level of effort and investment required. Accordingly, such amounts will be recognized as revenue in full in the period in which the associated milestone is achieved, assuming all other revenue recognition criteria are met. All commercial milestones will be accounted for in the same manner as royalties and recorded as revenue upon achievement of the milestone, assuming all other revenue recognition criteria are met.

During the years ended December 31, 2017, 2016, and 2015, the Company recognized $10.1 million, $14.2 million, and $17.3 million, respectively, of revenue associated with its collaboration with Sanofi Genzyme related to research and development services performed during the period and for consideration allocated to the PD Option, which was recognized during 2017. As of December 31, 2017, there is $31.6 million of deferred revenue related to the Sanofi Genzyme Collaboration Agreement, which is classified as either current or noncurrent in the accompanying balance sheet based on the period the services are expected to be delivered.

Costs incurred relating to the programs that Sanofi Genzyme has the option to license under the Sanofi Genzyme Collaboration Agreement consist of internal and external research and development costs, which primarily include: salaries and benefits, lab supplies and preclinical research studies. All costs are included in research and development expenses in the Company’s statement of operations during the year ended December 31, 2017. The Company estimates that the majority of research and development expense during the period relate to programs for which Sanofi Genzyme had or has an option right.

MRI Interventions License and Securities Purchase Agreements

Summary of Agreement

In September 2016, the Company entered into securities purchase and license agreements with MRI Interventions, Inc. (“MRIC”). MRIC is the primary supplier of the ClearPoint System, which is being used by the Company in ongoing development and clinical trials. The Company paid $2.0 million for shares of MRIC common stock and a warrant to purchase additional shares of MRIC common stock under a securities purchase agreement. The license agreement provided for certain rights to MRIC technology and for MRIC to transfer the rights and know-how to manufacture the ClearPoint System to enable the Company to utilize an alternative supplier for the ClearPoint System for use in the Company’s development and clinical trials.

Accounting Analysis

During the three months ended March 31, 2017, the Company terminated the license agreement with MRIC, and all prior and future commitments and obligations under such agreement became null and void. As of December 31, 2017, the Company continued to hold the common stock and warrants to purchase additional shares of common stock as an available-for-sale security and non-current asset, respectively.

Other Agreements

During 2017, 2016, and 2015, the Company entered into various agreements with contract research organizations and institutions to license intellectual property. In consideration for the licensed rights the Company generally made upfront payments, which were recorded as research and development expense as the acquired technologies were considered in‑process research and development. During the years ended December 31, 2017, 2016, and 2015, the Company paid $0.3 million, $0.6 million, and $0.1 million, respectively, in upfront license fees. The license agreements also obligate the Company to make additional payments that are contingent upon specific clinical trial and regulatory approval milestones being achieved as well as royalties on future product sales. The agreements to license intellectual property include potential milestone payments that are dependent upon the development of products licensed under the agreements and contingent upon the achievement of clinical trial or regulatory approval milestones. The maximum aggregate potential milestone payments payable by the Company total approximately $18.0 million. Additionally, under the terms of one agreement, the Company has options to license intellectual property to be used in the development of therapies for four additional disease indications. If the Company exercises all of the options under the agreement, it would be obligated to pay aggregate upfront fees of up to approximately $1.6 million and milestone payments that are contingent upon clinical trial results and regulatory approval of $5.0 million per disease indication, or up to $20.0 million in total. As of December 31, 2017 and 2016, there have been no milestones achieved. The Company can generally terminate the license agreements upon 30‑90 days prior written notice.

Additionally, certain license agreements require the Company to reimburse the licensor for certain past and ongoing patent related expenses. During the year ended December 31, 2017, 2016, and 2015, the Company incurred $0.8 million, $1.8 million, and $0.3 million of expense, respectively, related to these reimbursable patent costs which are recorded as general and administrative expense

During September 2016, the Company entered into a research and development funding arrangement with a non-profit organization that provides up to $4.0 million in funding to the Company upon the achievement of clinical and development milestones. The agreement provides that the Company repay amounts received under certain circumstances including termination of the agreement, and to pay an amount up to 2.6 times the funding received upon successful development and commercialization of any products developed. During 2017, the Company earned a milestone payment of $1.0 million. The Company has evaluated the arrangement and has concluded that it represents a research and development financing arrangement as it is probable that the Company will repay amounts received under the arrangement. As a result, the $1.0 million earned through 2017 is recorded as a non-current liability in the consolidated balance sheet.

Litigation

The Company is not a party to any litigation and does not have contingency reserves established for any litigation liabilities as of December 31, 2017 or December 31, 2016.

Redeemable Convertible Preferred Stock	12 Months Ended
Redeemable Convertible Preferred Stock	Dec. 31, 2017
Redeemable Convertible Preferred Stock Disclosure [Abstract]
Redeemable Convertible Preferred Stock [Text Block]	8. Redeemable convertible preferred stock In November 2015, upon the closing of the Company’s IPO, all issued and outstanding redeemable convertible preferred stock was automatically converted into 17,647,054 shares of common stock, see Note 2. The Company’s redeemable convertible preferred stock (“Preferred Stock”), has been classified as temporary equity on the accompanying balance sheets instead of in stockholders’ deficit in accordance with authoritative guidance for the classification and measurement of redeemable securities as the redeemable convertible preferred stock is redeemable at the option of the holder after the redemption date, February 2021. The Company has authorized preferred stock amounting to 5,000,000 shares as of December 31, 2017 and 2016. The authorized preferred stock was classified under stockholders’ equity at December 31, 2017. Series A Preferred Stock The Company issued 45,000,000 shares of Series A Preferred Stock during 2014 and 2015. These shares were issued at various closings in 2014 and 2015 for $1.00 per share. The shares were issued in exchange for cash proceeds of $42.0 million, net of issuance costs of $32.0 thousand, and the exchange of outstanding redeemable Convertible Notes, including accrued interest, of approximately $2.9 million. Tranche Rights Issued with Series A Preferred Stock Included in the terms of the January 2014 Series A Preferred Stock Purchase Agreement were certain rights (“Tranche Rights”), granted to the investors of Series A Preferred Stock purchased in January 2014, including the holders of the redeemable Convertible Notes who exchanged the redeemable Convertible Notes. The Tranche Rights obligated the investors in Series A Preferred Stock to purchase and the Company to sell an additional 18,500,000 shares of Series A Preferred Stock at $1.00 per share contingent upon successful near term in licensing and progress on initial experiments and research and development planning (“Tranche Right I”). In addition, the investors were obligated to purchase, and the Company was obligated to sell an additional 20,000,000 shares of Series A Preferred Stock upon the development of project engine and achievement of certain clinical milestones (“Tranche Right II”). In addition, the Tranche Rights allowed the investors the ability to purchase the additional shares at their option at any time. The Tranche Rights were transferrable by the investors, subject to approval by the Board. The Company has concluded the Tranche Rights meet the definition of a freestanding financial instrument, as the Tranche Rights were legally detachable and separately exercisable from the Series A Preferred Stock. Therefore, the Company allocated the proceeds between the Tranche Rights and the Series A Preferred Stock. As the Series A Preferred Stock was redeemable at the holder’s option, the Tranche Rights were classified as an asset or liability and were initially recorded at fair value. The Tranche Rights were measured at fair value at each reporting period. Since the Tranche Rights were subject to fair value accounting, the Company allocated the proceeds to the Tranche Rights based on the fair value at the date of issuance with the remaining proceeds being allocated to the Series A Preferred Stock. The estimated fair value of the Tranche Rights was determined using a probability weighted present value model that considers the probability of closing a tranche, the estimated future value of Series A Preferred Stock each closing, and the investment required at each closing. Future values were converted to present value using a discount rate appropriate for probability adjusted cash flows. Tranche Right I was initially recorded as an asset of $1.5 million as the purchase price of the additional shares was greater than the estimated value of the Series A Preferred Stock at the expected settlement date. The Company issued 18,500,000 additional shares under Tranche Right I, in three separate closings during the year ended December 31, 2014 with total proceeds of $18.5 million, net of issuance costs. Prior to each closing, any change in the value of Tranche Right I was recorded as other financing expense. The fair value of the portion of the Tranche Right I settled at each closing was reclassified to Series A Preferred Stock. Tranche Right II was initially recorded as a liability of $4.1 million as the purchase price of the additional shares was less than the estimated price of the Series A Preferred Stock at the expected settlement date. In February 2015, Tranche Right II was settled when the Company closed the final issuance of Series A Preferred Stock. The Company recognized expense of $9.8 million related to the mark to market of Tranche Right II during the year ended December 31, 2015, which was included in other financing expense. The fair value of the Tranche Right II settled at closing was reclassified to Series A Preferred Stock. The initial carrying amount of the Series A Preferred Stock issued upon the closing of Tranche Right II amounted to approximately $36.1 million which exceeded the redemption value, therefore the carrying value was not subsequently adjusted. However, the Company reflected accrued dividends of approximately $1.2 million related to this issuance in the net loss attributable to common stockholders for the year ended December 31, 2015. Series B Preferred Stock The Company issued 30,000,001 shares of Series B Preferred Stock during 2015. These shares were issued for $3.00 per share. This issuance resulted in cash proceeds of $89.8 million, net of issuance costs of $0.2 million. Additionally, a discount of $5.0 million was recorded against the proceeds as the amount paid by Sanofi Genzyme was in excess of fair value of the Series B Preferred Stock at issuance.

Common Stock

12 Months Ended

Dec. 31, 2017

Common stock Abstract

Stockholders' Equity Note Disclosure [Text Block]

9. Common stock

As of December 31, 2017 and 2016, the Company had authorized 120,000,000 shares of Common Stock, at $0.001 par value per share.

General

The voting, dividend and liquidation rights of the holders of the Common Stock are subject to and qualified by the rights, powers and preferences of the holders of Preferred Stock. The Common Stock has the following characteristics:

Voting

The holders of shares of Common Stock are entitled to one vote for each share of Common Stock held at all meetings of stockholders and written actions in lieu of meetings.

Dividends

The holders of shares of Common Stock are entitled to receive dividends, if and when declared by the Board of Directors. No dividends have been declared or paid by the Company since its inception.

Liquidation

The holders of shares of Common Stock are entitled to share ratably in the Company’s remaining assets available for distribution to its stockholders in the event of any voluntary or involuntary liquidation, dissolution or winding up of the Company or upon occurrence of a deemed liquidation event.

Shares Reserved For Future Issuance


	As of December 31,
	2017	2016
Shares reserved for vesting of restricted stock awards under the Founder Agreements	366,914	628,679
Shares reserved for vesting of restricted stock awards under the 2014 Option and Stock Plan	191,065	539,305
Shares reserved for exercise of stock options	3,143,566	1,871,237
Shares reserved for issuances under the 2015 Stock Option Plan	1,501,005	1,825,174
Shares reserved for issuances under the 2015 Employee Stock Purchase Plan	730,860	529,854
	5,933,410	5,394,249

Stock-based compensation

12 Months Ended

Dec. 31, 2017

Stock based compensation

Disclosure of Compensation Related Costs, Share-based Payments [Text Block]

10. Stock‑based compensation

2015 Stock Option Plan

In October 2015, the Company’s board of directors and stockholders approved the 2015 Stock Option and Incentive Plan “(2015 Stock Option Plan”), which became effective upon the completion of the IPO. The 2015 Stock Option Plan provides the Company with the flexibility to use various equity-based incentive and other awards as compensation tools to motivate its workforce. These tools include stock options, stock appreciation rights, restricted stock, restricted stock units, unrestricted stock, performance share awards and cash-based awards. The 2015 Stock Option Plan replaced the 2014 Plan (as defined below). Any options or awards outstanding under the 2014 Plan remained outstanding and effective. The number of shares initially reserved for issuance under the 2015 Stock Option Plan is the sum of (i) 1,311,812 shares of common stock and (ii) the number of shares under the 2014 Plan that are not needed to fulfill the Company’s obligations for awards issued under the 2014 Plan as a result of forfeiture, expiration, cancellation, termination or net issuances of awards thereunder. The number of shares of common stock that may be issued under the 2015 Stock Option Plan is also subject to increase on the first day of each fiscal year by up to 4% of the Company’s issued and outstanding shares of common stock on the immediately preceding December 31.

Effective January 1, 2016, 2017, and 2018 an additional 1,069,971, 1,070,635, and 1,285,200 shares, respectively, were added to the Company’s 2015 Stock Option Plan for future issuance. During the year ended December 31, 2017, the Company issued 1,700,000 stock options to employees and directors and 20,000 stock options to non‑employees. As of December 31, 2017, there were 1,501,005 shares available for future issuance under the 2015 Stock Option Plan.

2014 Stock Option and Grant Plan

In January 2014, the Company adopted the 2014 Stock Option and Grant Plan (the “2014 Plan”), under which it could grant incentive stock options, non‑qualified stock options, restricted stock awards, unrestricted stock awards, or restricted stock units to purchase up to 823,529 shares of Common Stock to employees, officers, directors and consultants of the Company.

In April 2014, the Company amended the Plan to allow for the issuance of up to 1,411,764 shares of Common Stock. In August 2014, April 2015, August 2015 and October 2015 the Company further amended the Plan to allow for the issuance of up to 2,000,000, 2,047,058, 2,669,411 and 2,998,823 shares of Common Stock, respectively. During 2014 the Company issued only restricted stock awards under the Plan and during 2015 the Company only granted stock options.

The terms of stock awards agreements, including vesting requirements, were determined by the Board of Directors and were subject to the provisions of the 2014 Plan. Restricted stock awards granted by the Company generally vest based on each grantee’s continued service with the Company during a specified period following grant. Awards granted to employees generally vest over four years, with 25% vesting on the one year anniversary and 75% vesting ratably, on a monthly basis, over the remaining three years. Awards granted to non‑employee consultants generally vest monthly over a period of one to four years.

Founder Awards

In January 2014, the Company issued 1,188,233 shares of restricted stock to its Founders at an original issuance price of $0.0425 per share. Of the total restricted shares awarded to the Founders, 835,292 shares generally vest over one to four years, based on each Founder’s continued service to the Company in varying capacity as a Scientific Advisory Board member, consultant, director, officer or employee, as set forth in each grantee’s individual restricted stock purchase agreement.

The remaining 352,941 of the shares issued will begin vesting upon the achievement of certain performance objectives as well as continued service to the Company, as set forth in the agreements. These performance conditions are tied to certain milestone events specific to the Company’s corporate goals, including but not limited to preclinical and clinical development milestones related to the Company’s product candidates. Stock‑based compensation expense associated with these performance‑based awards will be recognized when the achievement of the performance condition is considered probable, using management’s best estimates. Management concluded that the achievement of the performance milestone for one of the three performance-based awards had been met during 2016. Accordingly, stock‑based compensation expense in the amount of $1.4 million and $1.1 million was recorded in the years ended December 31, 2017 and 2016, respectively. No stock-based compensation expense was recorded related to the performance-based awards in the year ended December 31, 2015.

2015 Employee Stock Purchase Plan

In October 2015, the Company’s board of directors and stockholders approved the 2015 Employee Stock Purchase Plan (the “2015 ESPP”). Under the 2015 ESPP, all full-time employees of the Company are eligible to purchase common stock of the Company twice per year, at the end of each six-month payment period. During each payment period, eligible employees who so elect, may authorize payroll deductions in an amount of 1% to 10% (whole percentages only) of the employee’s base pay for each payroll period. At the end of each payment period, the accumulated deductions are used to purchase shares of common stock from the Company at a discount. A total of 262,362 shares of common stock were initially authorized for issuance under this plan. The 2015 ESPP became effective upon the completion of the IPO. Effective January 1, 2016, 2017, and 2018, a total of 267,492, 267,658, and 321,300 shares of common stock, respectively, were added to the 2015 ESPP. The Company issued 66,652 shares of common stock under the 2015 ESPP in the year ended December 31, 2017.

Stock-based Compensation Expense

Total compensation cost recognized for all stock-based compensation awards in the statements of operations and comprehensive loss is as follows:


	Year ended December 31,
	2017		2016		2015
	(in thousands)
Research and development	$	5,367	$	4,296	$	3,218
General and administrative		3,871		2,014		809
Total stock-compensation expense	$	9,238	$	6,310	$	4,027

Restricted Stock

A summary of the status of and changes in unvested restricted stock was as follows:


		Weighted
		Average
		Grant Date
		Fair Value
	Shares	Per Share
Unvested restricted common stock as of December 31, 2016	1,167,984	$	0.76
Issued	—
Vested	(573,803)	$	0.77
Repurchased	(36,202)	$	0.67
Unvested restricted common stock as of December 31, 2017	557,979	$	0.70

The expense related to awards granted to employees and non‑employees was $0.5 million and $2.7 million, respectively, for the year ended December 31, 2017. The expense related to awards granted to employees and non-employees was $0.5 million and $2.6 million, respectively, for the year ended December 31, 2016. The expense related to awards granted to employees and non-employees was $0.5 million and $2.6 million, respectively, for the year ended December 31, 2015.

As of December 31, 2017, the Company had unrecognized stock‑based compensation expense related to its unvested restricted stock awards of $4.5 million, which is expected to be recognized over the remaining weighted average vesting period of 0.38 years.

Stock Options

A summary of the status of, and changes in, stock options was as follows:


		Weighted		Remaining	Aggregate
		Average		Contractual	Intrinsic
		Exercise		Life	Value
	Shares	Price		(in years)	(in thousands)
Outstanding at December 31, 2016	1,871,237	$	10.21
Granted	1,720,000	$	13.07
Exercised	(158,677)	$	8.59
Cancelled or forfeited	(288,994)	$	10.50
Outstanding at December 31, 2017	3,143,566	$	11.82	8.6	$	15,991
Exercisable at December 31, 2017	955,587	$	10.45	8.1	$	5,933
Vested and expected to vest at December 31, 2017	3,143,566	$	11.82	8.6	$	15,991

Using the Black‑Scholes option pricing model, the weighted average fair value of options granted to employees and directors during the year ended December 31, 2017 was $8.48. The stock-based compensation expense related to stock option awards granted to employees and directors was $5.5 million, $3.0 million, and $0.7 million for the years ended December 31, 2017, 2016, and 2015, respectively.

The fair value of each option issued to employees and directors was estimated at the date of grant using the Black‑Scholes option pricing model with the following weighted‑average assumptions:


	Year ended December 31,
	2017		2016		2015
Risk-free interest rate	2.0	%	1.5	%	1.6	%
Expected dividend yield	—	%	—	%	—	%
Expected term (in years)	6.0		6.0		6.0
Expected volatility	73.7	%	73.1	%	78.6	%

Using the Black‑Scholes option pricing model, the weighted average grant date fair value of options granted to non‑employees during the year ended December 31, 2017 was $6.88. Unvested options granted to non‑employees are revalued at each measurement period until they vest. The expense related to stock option awards granted to non‑employees was $0.4 million, $0.2 million, and $0.3 million for the years ended December 31, 2017, 2016, and 2015, respectively. As of December 31, 2017, the Company had unrecognized stock‑based compensation expense related to its unvested stock options of $16.1 million which is expected to be recognized over the remaining weighted average vesting period of 2.66 years.

The fair value of each option issued to non‑employees was estimated at each vesting and reporting date using the Black‑Scholes option pricing model. The reporting date fair value was determined using the following weighted‑average assumptions:


	As of December 31,
	2017		2016		2015
Risk-free interest rate	2.4	%	2.1	%	2.0	%
Expected dividend yield	—	%	—	%	—	%
Expected term (in years)	8.5		9.1		10.0
Expected volatility	76.2	%	83.3	%	84.0	%

401(k) Savings Plan	12 Months Ended
401(k) Savings Plan	Dec. 31, 2017
401(k) Savings Plan
401(k) Savings Plan	11. 401(k) Savings plan The Company has a defined‑contribution savings plan under Section 401(k) of the Internal Revenue Code (the “401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements, and allows participants to defer a portion of their annual compensation on a pretax basis. The Company expensed approximately $0.5 million and $0.3 million related to employer contributions made during the years ended December 31, 2017 and 2016, respectively. The Company did not make any contributions to the 401(k) Plan during 2015.

Income taxes

12 Months Ended

Dec. 31, 2017

Income taxes

Income taxes

12. Income taxes

The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the Company’s financial statements or tax returns. Under this method, deferred tax assets and liabilities are determined based on differences between the financial statement carrying amounts and the tax bases of the assets and liabilities using the enacted tax rates in effect in the years in which the differences are expected to reverse. A valuation allowance against deferred tax assets is recorded if, based on the weight of the available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. The Company accounts for uncertain tax positions using a more-likely-than-not threshold for recognizing and resolving uncertain tax positions. The evaluation of uncertain tax positions is based on factors including, but not limited to, changes in the law, the measurement of tax positions taken or expected to be taken in tax returns, the effective settlement of matters subject to audit, new audit activity, and changes in facts or circumstances related to a tax position. The Company evaluates its tax positions on an annual basis.

On December 22, 2017, legislation commonly known as the Tax Cuts and Jobs Act (the “Tax Act”) was signed into law. The Tax Act, among other changes, reduces the U.S. federal corporate tax rate from 34% to 21%, requires taxpayers to pay a one-time transition tax on earnings of certain foreign subsidiaries that were previously tax deferred and creates new taxes on certain foreign sourced earnings. As of December 31, 2017, the Company did not have any foreign subsidiaries and the international aspects of the Tax Act are not applicable.

In connection with the initial analysis on the impact of the Tax Act, the Company remeasured certain deferred tax assets and liabilities based on the rates at which they are expected to reverse in the future, which is generally 21%. The remeasurement of the Company's deferred tax balance was primarily offset by application of its valuation allowance. However, the reduction of the U.S. federal corporate tax rate resulted in increases to the amounts reflected in “Deferred Rate Change” and “Change in valuation allowance” captions for the year ended December 31, 2017 in the Company’s tax reconciliation table compared to those amounts disclosed for the years ended December 31, 2016 and 2015. The change in the U.S. federal corporate tax rate, which is effective January 1, 2018, is also reflected in the Company’s deferred tax table.

The Company is still in the process of analyzing the impact to the Company of the Tax Act. On December 22, 2017, the SEC staff issued SAB 118 to address the application of GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the Tax Act. The Company has recognized the provisional tax impacts related to the revaluation of the deferred tax assets and liabilities and included these amounts in its consolidated financial statements for the year ended December 31, 2017. The ultimate impact may differ from these provisional amounts due to, among other things, additional analysis, changes in interpretations and assumptions the Company has made, additional regulatory guidance that may be issued, and actions the Company may take as a result of the Tax Act, which could result in changes to the provisional tax impacts during 2018.

The provision for income taxes is as follows:


	Year ended December 31,
	2017		2016
	(in thousands)
Current
Federal	$	—	$	180
State		—		—
Total current		—		180
Deferred
Federal		—		(111)
State		—		(17)
Total deferred		—		(128)
Total tax expense	$	—	$	52

A reconciliation of the expected income tax (benefit) computed using the federal statutory income tax rate at the Company’s effective tax rate is as follows:


	Year ended December 31,
	2017			2016			2015
Income tax computed at federal statutory tax rate		34.0	%		34.0	%		34.0	%
State taxes, net of federal benefit		6.1	%		5.6	%		4.1	%
General business credit carryovers		5.0	%		4.2	%		3.1	%
Non-deductible expenses		(4.1)	%		(4.0)	%		(15.5)	%
Deferred rate change		(21.8)	%		—	%		—	%
Change in valuation allowance		(19.2)	%		(40.2)	%		(25.7)	%
Total		—	%		(0.4)	%		—	%

The Company has incurred net operating losses (“NOLs”) since June 2013. At December 31, 2017, the Company had federal and state net operating loss carryforwards of $89.8 million and $91.7 million, respectively, which expire beginning in 2033. As of December 31, 2017, the Company also had federal and state research and development tax credit carryforwards of $6.6 million and $2.3 million, respectively, which expire beginning in 2028. As of December 31, 2017, the Company had state investment credits of $0.3 million, which expire beginning in 2018.

Under the provisions of the Internal Revenue Code, certain substantial changes in the Company’s ownership may result in a limitation on the amount of NOL carryforwards and research and development credit carryforwards that may be utilized annually to offset future taxable income and taxes payable. In general, an ownership change, as defined by Section 382, results from transactions that increase the ownership of 5% stockholders or public groups in the stock of a corporation by more than 50 percent in the aggregate over a three-year period. During 2016, the Company completed a study through June 30, 2016, to determine whether any ownership change has occurred since the Company’s formation and has determined that transactions have resulted in three ownership changes, as defined by Section 382. There could be additional ownership changes in the future that could further limit the amount of NOLs and tax credit carryforwards that the Company can utilize.

The significant components of the Company’s deferred tax assets and (liabilities) as of December 31, 2017 and 2016 are as follows:


	Year ended December 31,
	2017		2016
	(in thousands)
Deferred tax assets:
Net operating loss carryforwards	$	24,642	$	8,927
Tax credit carryforwards		8,832		4,284
Deferred rent		1,458		1,964
Deferred revenue		8,622		16,333
Non-deductible expenses		817		714
Intangibles		832		998
Stock compensation		1,361		672
Total deferred tax assets		46,564		33,892
Less valuation allowance		(44,953)		(31,361)
Net deferred tax assets		1,611		2,531
Deferred tax liabilities
Depreciation and amortization		(1,611)		(2,501)
Unrealized gain on available-for-sale securities		—		(30)
Net deferred taxes	$	—	$	—

As required by ASC 740, management has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets, which principally comprise NOL carryforwards, research and development credit carryforwards, and capitalized license and organization costs. Management has determined that it is more likely than not that the Company will not recognize the benefits of its federal and state deferred tax assets, and as a result, a valuation allowance of $45.0 million and $31.4 million has been established at December 31, 2017 and 2016, respectively. The change in valuation allowance was $13.6 million for the year ended December 31, 2017, primarily due to additional operating losses incurred by the Company for the year ended December 31, 2017, partially offset by the federal rate reduction from 34% to 21% as a result of the Tax Act. The primary reason for the difference between the income tax expense recorded by the Company and the amount of income tax expense at statutory income tax rates was the change in the valuation allowance.

At December 31, 2017 and 2016, the Company had no unrecognized tax benefits. The Company has not as yet conducted a study of its research and development credit carryforwards. This study may result in an adjustment to the Company’s research and development credit carryforwards; however, until a study is completed, and any adjustment is known, no amounts are being presented as an uncertain tax position. A full valuation allowance has been provided against the Company’s research and development credits, and if an adjustment is required, this adjustment would be offset by an adjustment to the valuation allowance. Thus, there would be no impact to the balance sheets or statements of operations if an adjustment were required.

Interest and penalty charges, if any, related to unrecognized tax benefits would be classified as income tax expense in the accompanying statements of operations. As of December 31, 2017 and 2016, the Company has no accrued interest related to uncertain tax positions. Since the Company is in a loss carryforward position, it is generally subject to examination by the U.S. federal, state, and local income tax authorities for all tax years in which a loss carryforward is available.

Related party transactions	12 Months Ended
Related party transactions	Dec. 31, 2017
Related party transactions
Related party transactions	13. Related‑party transactions Since inception, the Company received consulting and management services from one of its investors. The total amount of consulting and management services provided by this investor was approximately $31.8 thousand, $0.1 million, and $1.3 million during the years ended December 31, 2017, 2016, and 2015, respectively. During the years ended December 31, 2017, 2016, and 2015, the Company recognized $10.1 million, $14.2 million, and $17.3 million, respectively, of revenue associated with its collaboration with Sanofi Genzyme related to research and development services provided during these periods. The Company also recognized $0.1 million, $1.2 million, and $2.3 million of expense during the years ended December 31, 2017, 2016, and 2015, respectively, related to in‑kind services provided by Sanofi Genzyme associated with the collaboration arrangement.

Selected quarterly financial data (unaudited)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data

14. Selected quarterly financial data (unaudited)

The following table contains quarterly financial information for 2017 and 2016. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period.


	2017
	First		Second		Third		Fourth
	Quarter		Quarter		Quarter		Quarter		Total
	(amounts in thousands, except per share data)
Collaboration revenue	$	1,464	$	1,177	$	1,148	$	6,346	$	10,135
Total operating expenses		18,986		19,816		24,503		18,693		81,998
Loss from operations		(17,522)		(18,639)		(23,355)		(12,347)		(71,863)
Net loss attributable to common stockholders		(16,648)		(18,876)		(23,346)		(11,828)		(70,698)
Net loss per share applicable to common stockholders – basic and diluted	$	(0.65)	$	(0.73)	$	(0.89)	$	(0.40)	$	(2.64)


	2016
	First		Second		Third		Fourth
	Quarter		Quarter		Quarter		Quarter		Total
	(amounts in thousands, except per share data)
Collaboration revenue	$	4,830	$	3,720	$	3,308	$	2,362	$	14,220
Total operating expenses		12,297		13,338		13,679		16,205		55,519
Loss from operations		(7,467)		(9,618)		(10,371)		(13,843)		(41,299)
Net loss attributable to common stockholders		(7,188)		(9,335)		(8,996)		(14,674)		(40,193)
Net loss per share applicable to common stockholders – basic and diluted	$	(0.29)	$	(0.37)	$	(0.35)	$	(0.57)	$	(1.59)

Subsequent events	12 Months Ended
Subsequent events	Dec. 31, 2017
Subsequent events
Subsequent events	15. Subsequent events Lease Amendment In February 2018, the Company executed an amendment to lease an additional approximately 11,000 square feet at 75 Sidney Street that is intended to support its continued growth. The additional facility includes laboratory and office space, and will be ready for occupancy in the first half of 2018. AbbVie Collaboration In February 2018, the Company entered into an exclusive collaboration and option agreement with AbbVie Biotechnology Ltd (“AbbVie”), for the research, development, and commercialization of AAV and other virus-based gene therapy products for the treatment of diseases of the central nervous system and other neurodegenerative diseases related to defective or excess aggregation of tau protein in the human brain, including Alzheimer’s disease. Under the terms of the agreement, the Company received an upfront payment of $69.0 million and may receive future development and regulatory milestone payments and royalties. Under the terms of the agreement, the Company will perform specified research, preclinical, and Phase 1 development activities regarding vectorized antibodies directed against tau, after which AbbVie may select one or more vectorized antibodies to proceed into IND-enabling studies and clinical development. The Company will be responsible for the research, IND-enabling, and Phase 1 clinical trial activities and costs. Following the completion of Phase 1 clinical development, AbbVie has an option to license the vectorized tau antibody program and would then lead further clinical development and global commercialization for the product candidates pursuant to the agreement. The Company has an option to share in the costs of clinical development for higher royalty rates. In addition to the upfront and potential option exercise payments, the Company is eligible to receive up to $895.0 million in development and regulatory milestones for each vectorized tau antibody compound. The Company is also eligible to receive tiered, escalating royalties in a range, subject to certain specified exceptions, from a high-single digit to a mid-to-high teen percentage of the global net sales of the vectorized antibodies for tauopathies, including Alzheimer's disease and other neurodegenerative diseases. Under the terms of the agreement, each party will own the entire right, title, and interest in and to all know-how and patent rights first made or invented solely by it or its affiliates or its or their sublicensees in the course of the collaboration, with certain specified exceptions. The Company has agreed to grant AbbVie worldwide license to certain know-how and patent rights developed by us or jointly by the parties arising from the collaboration.

Summary of significant accounting policies (Policies)

12 Months Ended

Dec. 31, 2017

Summary of significant accounting policies

Basis of presentation

Public offerings

Use of estimates

Fair Value of Financial Instruments

Level 1—Quoted market prices in active markets for identical assets or liabilities.

Level 2—Inputs other than Level 1 inputs that are either directly or indirectly observable, such as quoted market prices, interest rates, and yield curves.

Level 3—Unobservable inputs developed using estimates of assumptions developed by the Company, which reflect those that a market participant would use.

Cash and cash equivalents

Marketable securities

Cash, cash equivalents, and marketable securities as of December 31, 2017 and 2016 consist of the following:


	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
	(in thousands)
As of December 31, 2017
Money market funds included in cash and cash equivalents	$	30,469	$	—	$	—	$	30,469
Marketable securities:
U.S. Treasury notes		137,560		—		38		137,522
Equity securities		1,220		—		120		1,100
Total marketable securities	$	138,780	$	—	$	158	$	138,622
Total money market funds and marketable securities	$	169,249	$	—	$	158	$	169,091
As of December 31, 2016
Money market funds included in cash and cash equivalents	$	36,003	$	—	$	—	$	36,003
Marketable securities:
U.S. Treasury notes		130,237		2		66		130,173
U.S. Government agency bonds		7,604		—		—		7,604
Total debt securities	$	137,841	$	2	$	66	$	137,777
Equity securities		1,220		140		—		1,360
Total marketable securities	$	139,061	$	142	$	66	$	139,137
Total money market funds and marketable securities	$	175,064	$	142	$	66	$	175,140

All of the Company’s marketable debt securities at December 31, 2017 and 2016 have a contractual maturity of one year or less.

Restricted cash

Property and equipment

Impairment of long-lived assets

Revenue Recognition

As of December 31, 2017, all of the Company’s revenue is generated exclusively from its collaboration agreement with Sanofi Genzyme Corporation, a Sanofi company (“Sanofi Genzyme”).

Persuasive evidence of an arrangement exists;

Delivery has occurred or services have been rendered;

The seller’s price to the buyer is fixed or determinable; and

Collectability is reasonably assured.

Research and development

Research contract costs and accruals

Patent costs

The Company expenses patent application and related legal costs as incurred and classifies such costs as general and administrative expenses in the accompanying statements of operations.

Stock-based compensation expense

Income taxes

Comprehensive loss

Comprehensive loss is comprised of net loss and other comprehensive income or loss. Other comprehensive gain or loss consists of unrealized gains or losses on marketable securities.

Net loss per share


	As of December 31,
	2017	2016	2015
Unvested restricted common stock	557,979	1,167,984	1,818,261
Shares reserved for issuance under stock plans	5,375,431	4,226,265	2,905,458
Total	5,933,410	5,394,249	4,723,719

Concentration of credit and off-balance sheet risk

Concentration of suppliers

Segment information

Recent accounting pronouncements

Summary of significant accounting policies (Tables)

12 Months Ended

Dec. 31, 2017

Summary of significant accounting policies

Schedule of Marketable securities

Cash, cash equivalents, and marketable securities as of December 31, 2017 and 2016 consist of the following:


	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
	(in thousands)
As of December 31, 2017
Money market funds included in cash and cash equivalents	$	30,469	$	—	$	—	$	30,469
Marketable securities:
U.S. Treasury notes		137,560		—		38		137,522
Equity securities		1,220		—		120		1,100
Total marketable securities	$	138,780	$	—	$	158	$	138,622
Total money market funds and marketable securities	$	169,249	$	—	$	158	$	169,091
As of December 31, 2016
Money market funds included in cash and cash equivalents	$	36,003	$	—	$	—	$	36,003
Marketable securities:
U.S. Treasury notes		130,237		2		66		130,173
U.S. Government agency bonds		7,604		—		—		7,604
Total debt securities	$	137,841	$	2	$	66	$	137,777
Equity securities		1,220		140		—		1,360
Total marketable securities	$	139,061	$	142	$	66	$	139,137
Total money market funds and marketable securities	$	175,064	$	142	$	66	$	175,140

Outstanding potentially dilutive securities excluded in the calculation of diluted net loss per share


	As of December 31,
	2017	2016	2015
Unvested restricted common stock	557,979	1,167,984	1,818,261
Shares reserved for issuance under stock plans	5,375,431	4,226,265	2,905,458
Total	5,933,410	5,394,249	4,723,719

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Measurements

Assets and liabilities measured at fair value on a recurring basis

Assets and liabilities measured at fair value on a recurring basis as of December 31, 2017 and 2016 are as follows:


			Quoted Prices		Significant
			in Active		Other		Significant
			Markets for		Observable		Unobservable
			Identical Assets		Inputs		Inputs
Assets	Total		(Level 1)		(Level 2)		(Level 3)
	(in thousands)
December 31, 2017
Money market funds included in cash and cash equivalents	$	30,469	$	30,469	$	—	$	—
Marketable securities:
U.S. Treasury notes		137,522		137,522		—		—
Equity securities		1,100		1,100		—		—
Total marketable securities	$	138,622	$	138,622	$	—	$	—
Warrants to purchase equity securities		569		—		569		—
Total	$	169,660	$	169,091	$	569	$	—
December 31, 2016
Money market funds included in cash and cash equivalents	$	36,003	$	36,003	$	—	$	—
Marketable securities:
U.S. Treasury notes		130,173		130,173		—		—
U.S. Government agency securities		7,604		—		7,604		—
Equity securities		1,360		1,360		—		—
Total marketable securities	$	139,137	$	131,533	$	7,604	$	—
Warrants to purchase equity securities		792		—		792		—
Total	$	175,932	$	167,536	$	8,396	$	—

Assumptions

The assumptions utilized to value the warrants to purchase equity securities as of December 31, 2017 and 2016 are as follows:


	As of December 31,
	2017		2016
Risk-free interest rate	2.0	%	1.8	%
Expected dividend yield	—	%	—	%
Expected term (in years)	3.7		4.7
Expected volatility	103.5	%	97.5	%

Prepaid expenses and other current assets (Tables)

12 Months Ended

Dec. 31, 2017

Prepaid expense and other current assets

Schedule of prepaid expense and other current assets

4. Prepaid expenses and other current assets

Prepaid expenses and other current assets consist of the following:


	As of December 31,
	2017		2016
	(in thousands)
Prepaid research and development contracts	$	1,330	$	1,094
Other current assets		766		541
Prepaid insurance		520		430
Accrued interest receivable		122		339
Tenant improvement receivable		—		1,964
Total	$	2,738	$	4,368

Property and equipment, net (Tables)

12 Months Ended

Dec. 31, 2017

Property and equipment, net

Schedule of property and equipment, net

Property and equipment, net consists of the following:


	As of December 31,
	2017		2016
	(in thousands)
Leasehold improvements	$	6,421	$	1,341
Laboratory equipment		5,262		3,306
Furniture and office equipment		1,565		526
Other		25		242
Construction in progress		—		3,873
Total property and equipment		13,273		9,288
Less: accumulated depreciation		(2,990)		(1,395)
Property and equipment, net	$	10,283	$	7,893

Accrued expenses (Tables)

12 Months Ended

Dec. 31, 2017

Accrued expenses

Schedule of accrued expenses

Accrued expenses consist of the following:


	As of December 31,
	2017		2016
	(in thousands)
Research and development costs	$	5,780	$	2,384
Employee compensation costs		3,383		2,399
Professional services		1,762		698
Accrued goods and services		388		842
Patent costs		120		89
Other		64		76
Total	$	11,497	$	6,488

Commitments and contingencies (Tables)

12 Months Ended

Dec. 31, 2017

Commitments and Contingencies

Future annual minimum lease payments

Future annual minimum lease payments at December 31, 2017 are as follows:


	Total Minimum
	Lease Payments
	(in thousands)
2018		3,290
2019		3,382
2020		3,762
2021		3,868
2022		3,977
2023+		8,295
	$	26,574

Allocable arrangement consideration allocation


Unit of Accounting	Amount
	(in thousands)
Research and Development Services for:
Huntington’s Program	$	15,662
Parkinson’s Program		6,648
Friedreich’s Ataxia Program		16,315
Spinal Muscular Atrophy Program		32,050
Future Program		2,464
Committee Obligations:
AJSC		147
DAC		227
License Option and related deliverables		5,743
Total	$	79,256

Common Stock (Tables)

12 Months Ended

Dec. 31, 2017

Common stock Abstract

Schedule of shares reserved for future issuance

Shares Reserved For Future Issuance


	As of December 31,
	2017	2016
Shares reserved for vesting of restricted stock awards under the Founder Agreements	366,914	628,679
Shares reserved for vesting of restricted stock awards under the 2014 Option and Stock Plan	191,065	539,305
Shares reserved for exercise of stock options	3,143,566	1,871,237
Shares reserved for issuances under the 2015 Stock Option Plan	1,501,005	1,825,174
Shares reserved for issuances under the 2015 Employee Stock Purchase Plan	730,860	529,854
	5,933,410	5,394,249

- Definition

Tabular disclosure of an entity's stock, including par or stated value per share, number and dollar amount of share subscriptions, shares authorized, shares issued, shares outstanding, number and dollar amount of shares held in an employee trust, dividend per share, total dividends, share conversion features, par value plus additional paid in capital, the value of treasury stock and other information necessary to a fair presentation, and EPS information. Stock by class includes common, convertible, and preferred stocks which are not redeemable or redeemable solely at the option of the issuer. Includes preferred stock with redemption features that are solely within the control of the issuer and mandatorily redeemable stock if redemption is required to occur only upon liquidation or termination of the reporting entity. If more than one issue is outstanding, state the title of each issue and the corresponding dollar amount; dollar amount of any shares subscribed but unissued and the deduction of subscriptions receivable there from; number of shares authorized, issued, and outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21506-112644

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 480
-SubTopic 10
-Section S99
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=65877616&loc=d3e177068-122764

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(d))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21553-112644

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21475-112644

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(28))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=65888546&loc=d3e21300-112643

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(27))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21521-112644

Reference 12: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21484-112644

Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21538-112644

Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21488-112644

+ Details

Stock based compensation (Tables)

12 Months Ended

Dec. 31, 2017

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Compensation cost recognized for all stock-based compensation awards


	Year ended December 31,
	2017		2016		2015
	(in thousands)
Research and development	$	5,367	$	4,296	$	3,218
General and administrative		3,871		2,014		809
Total stock-compensation expense	$	9,238	$	6,310	$	4,027

Summary of status and changes in unvested restricted stock


		Weighted
		Average
		Grant Date
		Fair Value
	Shares	Per Share
Unvested restricted common stock as of December 31, 2016	1,167,984	$	0.76
Issued	—
Vested	(573,803)	$	0.77
Repurchased	(36,202)	$	0.67
Unvested restricted common stock as of December 31, 2017	557,979	$	0.70

Summary of status and changes in stock options


		Weighted		Remaining	Aggregate
		Average		Contractual	Intrinsic
		Exercise		Life	Value
	Shares	Price		(in years)	(in thousands)
Outstanding at December 31, 2016	1,871,237	$	10.21
Granted	1,720,000	$	13.07
Exercised	(158,677)	$	8.59
Cancelled or forfeited	(288,994)	$	10.50
Outstanding at December 31, 2017	3,143,566	$	11.82	8.6	$	15,991
Exercisable at December 31, 2017	955,587	$	10.45	8.1	$	5,933
Vested and expected to vest at December 31, 2017	3,143,566	$	11.82	8.6	$	15,991

Assumptions

The assumptions utilized to value the warrants to purchase equity securities as of December 31, 2017 and 2016 are as follows:


	As of December 31,
	2017		2016
Risk-free interest rate	2.0	%	1.8	%
Expected dividend yield	—	%	—	%
Expected term (in years)	3.7		4.7
Expected volatility	103.5	%	97.5	%

Employees and directors stock options

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Assumptions

The fair value of each option issued to employees and directors was estimated at the date of grant using the Black‑Scholes option pricing model with the following weighted‑average assumptions:


	Year ended December 31,
	2017		2016		2015
Risk-free interest rate	2.0	%	1.5	%	1.6	%
Expected dividend yield	—	%	—	%	—	%
Expected term (in years)	6.0		6.0		6.0
Expected volatility	73.7	%	73.1	%	78.6	%

Non-employees stock options

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Assumptions


	As of December 31,
	2017		2016		2015
Risk-free interest rate	2.4	%	2.1	%	2.0	%
Expected dividend yield	—	%	—	%	—	%
Expected term (in years)	8.5		9.1		10.0
Expected volatility	76.2	%	83.3	%	84.0	%

Income taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income taxes

Provision for income taxes

The provision for income taxes is as follows:


	Year ended December 31,
	2017		2016
	(in thousands)
Current
Federal	$	—	$	180
State		—		—
Total current		—		180
Deferred
Federal		—		(111)
State		—		(17)
Total deferred		—		(128)
Total tax expense	$	—	$	52

Schedule of reconciliation of the expected income tax (benefit) computed using the federal statutory income tax rate to the effective income tax rate

A reconciliation of the expected income tax (benefit) computed using the federal statutory income tax rate at the Company’s effective tax rate is as follows:


	Year ended December 31,
	2017			2016			2015
Income tax computed at federal statutory tax rate		34.0	%		34.0	%		34.0	%
State taxes, net of federal benefit		6.1	%		5.6	%		4.1	%
General business credit carryovers		5.0	%		4.2	%		3.1	%
Non-deductible expenses		(4.1)	%		(4.0)	%		(15.5)	%
Deferred rate change		(21.8)	%		—	%		—	%
Change in valuation allowance		(19.2)	%		(40.2)	%		(25.7)	%
Total		—	%		(0.4)	%		—	%

Schedule of principal components of the Company’s deferred tax assets and liabilities

The significant components of the Company’s deferred tax assets and (liabilities) as of December 31, 2017 and 2016 are as follows:


	Year ended December 31,
	2017		2016
	(in thousands)
Deferred tax assets:
Net operating loss carryforwards	$	24,642	$	8,927
Tax credit carryforwards		8,832		4,284
Deferred rent		1,458		1,964
Deferred revenue		8,622		16,333
Non-deductible expenses		817		714
Intangibles		832		998
Stock compensation		1,361		672
Total deferred tax assets		46,564		33,892
Less valuation allowance		(44,953)		(31,361)
Net deferred tax assets		1,611		2,531
Deferred tax liabilities
Depreciation and amortization		(1,611)		(2,501)
Unrealized gain on available-for-sale securities		—		(30)
Net deferred taxes	$	—	$	—

Selected quarterly financial data (unaudited) (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Schedule of Quarterly Financial Information


	2017
	First		Second		Third		Fourth
	Quarter		Quarter		Quarter		Quarter		Total
	(amounts in thousands, except per share data)
Collaboration revenue	$	1,464	$	1,177	$	1,148	$	6,346	$	10,135
Total operating expenses		18,986		19,816		24,503		18,693		81,998
Loss from operations		(17,522)		(18,639)		(23,355)		(12,347)		(71,863)
Net loss attributable to common stockholders		(16,648)		(18,876)		(23,346)		(11,828)		(70,698)
Net loss per share applicable to common stockholders – basic and diluted	$	(0.65)	$	(0.73)	$	(0.89)	$	(0.40)	$	(2.64)


	2016
	First		Second		Third		Fourth
	Quarter		Quarter		Quarter		Quarter		Total
	(amounts in thousands, except per share data)
Collaboration revenue	$	4,830	$	3,720	$	3,308	$	2,362	$	14,220
Total operating expenses		12,297		13,338		13,679		16,205		55,519
Loss from operations		(7,467)		(9,618)		(10,371)		(13,843)		(41,299)
Net loss attributable to common stockholders		(7,188)		(9,335)		(8,996)		(14,674)		(40,193)
Net loss per share applicable to common stockholders – basic and diluted	$	(0.29)	$	(0.37)	$	(0.35)	$	(0.57)	$	(1.59)

Nature of business (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
Nature of business (Details) - USD ($) $ in Thousands	Feb. 28, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net loss		$ (70,698)	$ (40,193)	$ (29,672)
Accumulated deficit		$ (160,713)	$ (90,015)
Collaborative Arrangement
Collaborative Arrangement Nonredeemable Up Front Fee Received	$ 65,000
Redeemable Convertible Preferred Stock Series B
Redeemable convertible preference stock issued (in shares)				30,000,001
Redeemable Convertible Preferred Stock Series B \| Collaborative Arrangement
Collaborative Arrangement Nonredeemable Up Front Fee Received	30,000
Genzyme \| Collaborative Arrangement
Collaborative Arrangement Nonredeemable Up Front Fee Received	$ 65,000
Genzyme \| Redeemable Convertible Preferred Stock Series B \| Collaborative Arrangement
Issuance of stock (in shares)	10,000,000

Summary of significant accounting policies - Basis of Presentation - (Details) $ / shares in Units, $ in Millions				12 Months Ended
	Nov. 07, 2017 USD ($) $ / shares shares	Nov. 16, 2015 USD ($) $ / shares shares	Oct. 29, 2015	Dec. 31, 2015 shares	Dec. 31, 2017 shares	Dec. 31, 2016 shares
Common stock sales	5,175,000				31,572,044	25,597,912
Price per share \| $ / shares	$ 12.00
Net proceeds \| $	$ 58.0
Reverse split ratio			0.235
Shares issued upon conversion of redeemable convertible preferred stock		17,647,054
IPO
Common stock sales		5,750,000
Price per share \| $ / shares		$ 14.00
Net proceeds \| $		$ 72.9
Common Stock
Shares issued upon conversion of redeemable convertible preferred stock				17,647,054

Summary of significant accounting policies - Marketable Securities - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Amortized cost basis
Cash and cash equivalents	$ 31,530	$ 36,641	$ 31,309	$ 7,035
Marketable debt securities, amortized cost	138,780	137,841
Total marketable securities, amortized cost		139,061
Total money market funds and marketable securities amortized cost	169,249	175,064
Unrealized gains
Debt securities, unrealized gains		2
Total marketable securities, unrealized gains	142
Total money market funds and marketable securities, unrealized gains	142
Unrealized losses
Debt securities, unrealised losses		66
Available-for-sale Securities, Gross Unrealized Loss, Total	158	66
Total money market funds and marketable securities	158	66
Fair value
Available-for-sale Securities, Debt Securities	138,622	137,777
Marketable securities, fair value		139,137
Total Money Market Funds And Marketable Securities Fair Value	169,091	175,140
Money market funds
Amortized cost basis
Cash and cash equivalents	30,469	36,003
Fair value
Cash and cash equivalents, fair value	30,469	36,003
Deposits and other non-current assets
Restricted Cash and Investments [Abstract]
Restricted cash	700
U.S. Treasury notes
Amortized cost basis
Marketable debt securities, amortized cost	137,560	130,237
Unrealized gains
Debt securities, unrealized gains		2
Unrealized losses
Debt securities, unrealised losses	38	66
Fair value
Available-for-sale Securities, Debt Securities	137,522	130,173
U. S. Government agency securities
Amortized cost basis
Marketable debt securities, amortized cost		7,604
Fair value
Available-for-sale Securities, Debt Securities		7,604
Equity Securities
Amortized cost basis
Marketable debt securities, amortized cost	1,220
Marketable equity securities, amortized cost		1,220
Unrealized gains
Equity securities unrealized gains		140
Unrealized losses
Equity securities, unrealized losses	120
Fair value
Available-for-sale Securities, Debt Securities	$ 1,100
Marketable equity securities, fair value		$ 1,360

Summary of significant accounting policies - Deferred Issuance Costs - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of significant accounting policies
Other comprehensive loss	$ (235)	$ 199	$ (251)

Summary of significant accounting policies - Net loss per share - (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Jan. 01, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share, Pro Forma [Abstract]
Antidilutive Securities Excluded from Computation of Earnings Per Share (in common stock equivalent shares)										5,933,410	5,394,249	4,723,719
Add: Changes in fair value of preferred stock tranche liability												$ 9,750
Net loss attributable to common stockholders		$ (11,828)	$ (23,346)	$ (18,876)	$ (16,648)	$ (14,674)	$ (8,996)	$ (9,335)	$ (7,188)	$ (70,698)	$ (40,193)	(38,290)
Accretion of redeemable convertible preferred stock to redemption value												(7,373)
Add: Accrued dividends on temporary equity												$ 1,245
Weighted average number of common shares outstanding, basic and diluted (in shares)										26,803,711	25,302,414	4,191,210
Increase in deferred revenue										$ (10,135)	$ (14,582)	$ 52,666
Minimum \| Forecast
Earnings Per Share, Pro Forma [Abstract]
Increase in deferred revenue	$ 18,000
Maximum \| Forecast
Earnings Per Share, Pro Forma [Abstract]
Increase in deferred revenue	$ 23,000
Unvested restricted stock awards
Earnings Per Share, Pro Forma [Abstract]
Antidilutive Securities Excluded from Computation of Earnings Per Share (in common stock equivalent shares)										557,979	1,167,984	1,818,261
Outstanding stock options
Earnings Per Share, Pro Forma [Abstract]
Antidilutive Securities Excluded from Computation of Earnings Per Share (in common stock equivalent shares)										5,375,431	4,226,265	2,905,458

Fair Value Measurements - Assets measured on a recurring basis - (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 02, 2016	Dec. 31, 2017
Fair value assumptions
Risk-free interest rate	1.80%	2.00%
Expected term (in years)	4 years 8 months 12 days	3 years 8 months 12 days
Expected volatility	97.50%	103.50%
Estimate of Fair Value
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	$ 139,137		$ 138,622
Warrants to purchase equity securities	792		569
Total assets measured at fair value	175,932		169,660
Level 1 \| Estimate of Fair Value
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	131,533		138,622
Total assets measured at fair value	167,536		169,091
Level 2 \| Estimate of Fair Value
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	7,604
Warrants to purchase equity securities	792		569
Total assets measured at fair value	8,396		569
U.S. Treasury notes \| Estimate of Fair Value
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	130,173		137,522
U.S. Treasury notes \| Level 1 \| Estimate of Fair Value
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	130,173		137,522
U. S. Government agency securities \| Estimate of Fair Value
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	7,604
U. S. Government agency securities \| Level 2 \| Estimate of Fair Value
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	7,604
Equity Securities \| Estimate of Fair Value
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	1,360		1,100
Equity Securities \| Level 1 \| Estimate of Fair Value
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	1,360		1,100
Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	36,003		30,469
Money market funds \| Estimate of Fair Value
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	36,003		30,469
Money market funds \| Level 1 \| Estimate of Fair Value
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 36,003		$ 30,469

Fair Value Measurements - Series A Preferred Stock - (Details)	Nov. 07, 2017 $ / shares
Fair Value Measurements
Estimated future value at closing	$ 12.00

Prepaid expenses and other current assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Prepaid expense and other current assets
Tenant improvement receivable	$ 1,330	$ 1,094
Prepaid expenses	766	541
Prepaid research and development contracts	520	430
Accrued interest receivable	122	339
Other current assets		1,964
Total	$ 2,738	$ 4,368

Property and equipment, net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 13,273	$ 9,288
Less: accumulated depreciation	(2,990)	(1,395)
Property and equipment, net	10,283	7,893
Depreciation expense	1,595	612	$ 600
Costs reimbursable	3,000
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	6,421	1,341
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	5,262	3,306
Furniture and office equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	1,565	526
Construction in Progress
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 25	242
Other
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross		$ 3,873

Accrued expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accrued expenses
Research and development costs	$ 5,780	$ 2,384
Employee compensation costs	3,383	2,399
Professional services	1,762	698
Accrued goods and services	388	842
Other	64	76
Patent costs	120	89
Total	$ 11,497	$ 6,488

Commitments and contingencies - Operating Leases - (Details) $ in Thousands	1 Months Ended		12 Months Ended
	Feb. 28, 2018 USD ($) ft²	Mar. 31, 2014 item	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Number of renewal options \| item		2
Rent expense			$ 2,000	$ 2,900	$ 900
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018			3,290
2019			3,382
2020			3,762
2021			3,868
2022			3,977
2023 +			8,295
Total minimum lease payments			26,574
75 Sidney Street
Additional space leased \| ft²	11,000
Term of each renewal option		5 years
Leasehold incentive received from landlord			1,300
64 Sidney Street
Leasehold incentive received from landlord			$ 3,500
Forecast
Leasehold improvement receivable	$ 300

Commitments and contingencies - Significant Agreements - (Details) - Collaborative Arrangement $ in Millions	1 Months Ended
	Feb. 28, 2015 USD ($) shares
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment received	$ 65.0
Premium over fair value on temporary equity issued	5.0
Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment received	65.0
Premium over fair value on temporary equity issued	5.0
Amount of reduction in milestone or royalty payments to be made by collaborative partner for each program specified	10.0
Per Milestone, maximum milestone payments to be received from collaborative partner	540.0
Regulatory approval milestone \| Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner	220.0
Regulatory approval milestone \| Minimum \| Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner	40.0
Regulatory approval milestone \| Maximum \| Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner	50.0
Commercial milestone \| Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner	320.0
Commercial milestone \| Minimum \| Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner	50.0
Commercial milestone \| Maximum \| Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner	100.0
Spinal Muscular Atrophy Program \| Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum potential in-kind services to be provided by collaborative partner	5.0
Option exercise payment required from collaborative partner for each program specified	30.0
Split Territory Program \| Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Option exercise payment required from collaborative partner for each program specified	20.0
Redeemable Convertible Preferred Stock Series B
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment received	30.0
Fair value of temporary equity issued	$ 25.0
Redeemable Convertible Preferred Stock Series B \| Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Temporary equity issued (in shares) \| shares	10,000,000

Commitments and contingencies - Accounting Analysis - (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended								12 Months Ended
	Sep. 30, 2016	Feb. 28, 2015	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allocable arrangement consideration allocation
Committee Obligations: Alliance Joint Steering Committee ("AJSC")		$ 147
Committee Obligations: Development Advisory Committee ("DAC")		227
License Option and related deliverables		5,743
Total allocable arrangement consideration allocations.		79,256
Income Statement Related Disclosures [Abstract]
Revenue adjustment due to development deprioritization											$ 9,500
Collaboration revenue			$ 6,346	$ 1,148	$ 1,177	$ 1,464	$ 2,362	$ 3,308	$ 3,720	$ 4,830	10,135	$ 14,220	$ 17,334
Huntington's Program
Allocable arrangement consideration allocation
Research and development services		15,662
Parkinson's Program
Allocable arrangement consideration allocation
Research and development services		6,648
Income Statement Related Disclosures [Abstract]
Revenue recognized											5,500
Friedreich's Ataxia Program
Allocable arrangement consideration allocation
Research and development services		16,315
Spinal Muscular Atrophy Program
Allocable arrangement consideration allocation
Research and development services		32,050
Future Program
Allocable arrangement consideration allocation
Research and development services		2,464
Collaborative Arrangement
Allocable arrangement consideration
Collaborative Arrangement Nonredeemable Up Front Fee Received		65,000
Additional consideration, premium over fair value on temporary equity issued		5,000
Genzyme
Income Statement Related Disclosures [Abstract]
Revenue adjustment due to development deprioritization											2,100
Genzyme \| Collaborative Arrangement
Allocable arrangement consideration
Collaborative Arrangement Nonredeemable Up Front Fee Received		65,000
Additional consideration, premium over fair value on temporary equity issued		5,000
In-kind services and other funding to be provided by collaborative partner		9,300
Total allocable arrangement consideration		79,300
Income Statement Related Disclosures [Abstract]
Collaboration revenue											10,100	$ 14,200	$ 17,300
Deferred Revenue			$ 31,600								$ 31,600
MRIC
Income Statement Related Disclosures [Abstract]
Payments for shares	$ 2,000
Redeemable Convertible Preferred Stock Series B \| Collaborative Arrangement
Allocable arrangement consideration
Collaborative Arrangement Nonredeemable Up Front Fee Received		$ 30,000

Commitments and contingencies - Other Agreements - (Details)	1 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($) multiple	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of additional disease indications for which the Company has options to license intellectual property \| item		4
Aggregate maximum milestone payments to be paid to collaborative partner		$ 18,000,000
Aggregate upfront fees to be paid to collaborative partner		$ 1,600,000
Number of milestones achieved \| item		0	0
Patent costs incurred		$ 800,000	$ 1,800,000	$ 300,000
Maximum funding	$ 4,000,000
Funding multiple \| multiple	2.60
Milestone payment liability		1,000,000
Loss Contingency [Abstract]
Contingency reserves for litigation liabilities		0
Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Per Milestone and per disease indication, maximum milestone payments to be paid to collaborative partner		$ 5,000,000
Number of days written notice required by reporting entity to terminate agreement		30 days
Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Per Milestone and per disease indication, maximum milestone payments to be paid to collaborative partner		$ 20,000,000
Number of days written notice required by reporting entity to terminate agreement		90 days
Various contract research organizations and institutions \| Research and development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Payments of upfront license fees		$ 300,000	$ 600,000	$ 100,000

Commitments and contingencies - UMass and MassBiologics Collaboration - (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2015
Common Stock
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Issuance of common stock (in shares)	5,175,000	5,750,000

Redeemable Convertible Preferred Stock - Series A Preferred Stock- (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended		24 Months Ended
	Nov. 16, 2015	Nov. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2017	Nov. 07, 2017	Dec. 31, 2016
Temporary Equity [Line Items]
Shares issued upon conversion of redeemable convertible preferred stock	17,647,054
Preferred stock, authorized						5,000,000		5,000,000
Price per share							$ 12.00
Issuance costs			$ 200
Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Shares issued upon conversion of redeemable convertible preferred stock		17,647,054
Redeemable Convertible Preferred Stock Series A
Temporary Equity [Line Items]
Redeemable convertible preference stock issued (in shares)			20,000,000		45,000,000
Price per share			$ 1.00	$ 1.00	$ 1.00
Proceeds from issuance of stock, net of issuance costs and exchange of outstanding convertible notes			$ 42,000	$ 42,000
Issuance costs			1		$ 32
Exchange of outstanding convertible notes and accrued interest			$ 2,900	$ 2,900
Redeemable Convertible Preferred Stock Series B
Temporary Equity [Line Items]
Redeemable convertible preference stock issued (in shares)			30,000,001
Price per share			$ 3.00		$ 3.00
Proceeds from issuance of stock, net of issuance costs			$ 89,800
Issuance costs			220
Discount recorded against proceeds received for temporary equity related to payment in excess over fair value			$ 5,000
Common Stock
Temporary Equity [Line Items]
Shares issued upon conversion of redeemable convertible preferred stock			17,647,054

Redeemable Convertible Preferred Stock - Tranche Rights Issued with Series A Preferred Stock - (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Nov. 07, 2017
Temporary Equity [Line Items]
Price per share				$ 12.00
Fair value of Tranche Rights
Change in fair value of preferred stock tranche liability		$ 9,750
Accrued dividends on temporary equity		$ 1,245
Tranche Right I
Fair value of Tranche Rights
Derivative Asset	$ 1,500
Issuance of stock (in shares)			18,500,000
Proceeds from issuance of stock, net of issuance costs			$ 18,500
Tranche Right II
Temporary Equity [Line Items]
Number of additional shares of temporary equity contingently obligated to be purchased by temporary equity holders	20,000,000
Fair value of Tranche Rights
Fair value of Tranche Right Liability			$ 4,100
Redeemable Convertible Preferred Stock Series A
Temporary Equity [Line Items]
Price per share		$ 1.00	$ 1.00
Fair value of Tranche Rights
Redeemable convertible preferred stock, initial carrying amount		$ 36,100
Redeemable Convertible Preferred Stock Series A \| Tranche Right I
Temporary Equity [Line Items]
Number of additional shares of temporary equity contingently obligated to be purchased by temporary equity holders	18,500,000
Price per share	$ 1.00
Redeemable Convertible Preferred Stock Series B
Temporary Equity [Line Items]
Price per share		$ 3.00
Fair value of Tranche Rights
Proceeds from issuance of stock, net of issuance costs		$ 89,800

Redeemable Convertible Preferred Stock - Series B Preferred Stock - (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Nov. 07, 2017
Temporary Equity [Line Items]
Price per share		$ 12.00
Issuance costs	$ 200
Redeemable Convertible Preferred Stock Series B
Temporary Equity [Line Items]
Redeemable convertible preference stock issued (in shares)	30,000,001
Price per share	$ 3.00
Proceeds from issuance of stock, net of issuance costs	$ 89,800
Issuance costs	220
Discount Recorded Against Proceeds Received For Temporary Equity Related To Excess Over Aggregate Fair Value	$ 5,000

Common Stock (Details)	12 Months Ended	35 Months Ended
Common Stock (Details)	Dec. 31, 2017 Vote $ / shares shares	Dec. 31, 2016 $ / shares shares	Oct. 31, 2015 shares	Dec. 31, 2014 shares
Class of Stock [Line Items]
Common Stock, Shares Authorized	120,000,000	120,000,000
Common stock, par value (in dollars per share) \| $ / shares	$ 0.001	$ 0.001
Number of votes entitled for each share of common stock \| Vote	1
Dividends declared \| $ / shares		0
Dividends paid \| $ / shares		$ 0
Shares available for future issuance (in shares)	5,933,410			5,394,249
2015 Stock Option Plan
Class of Stock [Line Items]
Shares available for future issuance (in shares)			1,311,812
Vesting of restricted stock awards \| Founder Awards
Class of Stock [Line Items]
Shares available for future issuance (in shares)	366,914			628,679
Vesting of restricted stock awards \| 2014 Stock Option and Stock Plan
Class of Stock [Line Items]
Shares available for future issuance (in shares)	191,065			539,305
Exercise of stock options
Class of Stock [Line Items]
Shares available for future issuance (in shares)	3,143,566			1,871,237
Issuance under share based compensation plans
Class of Stock [Line Items]
Shares available for future issuance (in shares)	730,860			529,854
Issuance under share based compensation plans \| 2015 Stock Option Plan
Class of Stock [Line Items]
Shares available for future issuance (in shares)	1,501,005			1,825,174

Stock based compensation - 2014 Stock Option and Grant Plan - (Details) $ / shares in Units, $ in Thousands				1 Months Ended		12 Months Ended
	Jan. 01, 2018 shares	Jan. 01, 2017 shares	Jan. 01, 2016 shares	Oct. 31, 2015 shares	Jan. 31, 2014 $ / shares shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Aug. 31, 2015 shares	Apr. 30, 2015 shares	Dec. 31, 2014 shares	Aug. 31, 2014 shares	Apr. 30, 2014 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)						5,933,410					5,394,249
Stock-based compensation \| $						$ 9,238	$ 6,310	$ 4,027
Unrecognized stock-based compensation expense \| $						$ 4,500
2014 Stock Option and Grant Plan (2014 Plan)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)						1,501,005
Number of shares authorized for stock-based compensation plan (in shares)				2,998,823	823,529				2,669,411	2,047,058		2,000,000	1,411,764
2015 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New stock options issued						66,652
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period						66,652
Number of shares authorized for stock-based compensation plan (in shares)		267,658	267,492	262,362
2015 Employee Stock Purchase Plan \| Forecast
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for stock-based compensation plan (in shares)	321,300
2015 Employee Stock Purchase Plan \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of payroll deduction				1.00
2015 Employee Stock Purchase Plan \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of payroll deduction				10.00
2015 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)				1,311,812
Percentage limit of issued and outstanding shares upto which additional shares may be issued				4.00
Additional shares added to stock option plan		1,070,635	1,069,971
2015 Stock Option Plan \| Forecast
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional shares added to stock option plan	1,285,200
Employees restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation \| $						$ 500	500	500
Non-employee consultants restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation \| $						2,700	2,600	2,600
Founders restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation \| $						1,400	1,100
Restricted stock granted (in shares)					1,188,233
Restricted stock granted, original issued price (in dollars per share) \| $ / shares					$ 0.0425
Founders restricted stock \| Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock granted (in shares)					835,292
Founders restricted stock \| Vesting period, tranche two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock granted (in shares)					352,941
Founders restricted stock \| Minimum \| Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					1 year
Founders restricted stock \| Maximum \| Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					4 years
Stock options, all inclusive
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock-based compensation expense \| $						$ 16,100
Stock options granted (in shares)						1,720,000
Employees and directors stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation \| $						$ 5,500	3,000	700
Employees and directors stock options \| 2014 Stock Option and Grant Plan (2014 Plan)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (in shares)						1,700,000
Vesting period						4 years
Employees and directors stock options \| 2014 Stock Option and Grant Plan (2014 Plan) \| Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period						1 year
Vesting percentage						25.00%
Employees and directors stock options \| 2014 Stock Option and Grant Plan (2014 Plan) \| Vesting period, tranche two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period						3 years
Vesting percentage						75.00%
Non-employees stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation \| $						$ 400	200	300
Non-employees stock options \| 2014 Stock Option and Grant Plan (2014 Plan)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (in shares)						20,000
Non-employees stock options \| 2014 Stock Option and Grant Plan (2014 Plan) \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period						1 year
Non-employees stock options \| 2014 Stock Option and Grant Plan (2014 Plan) \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period						4 years
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation \| $						$ 5,367	4,296	3,218
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation \| $						$ 3,871	$ 2,014	$ 809

Stock based compensation - Restricted Stock - (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted stock disclosures
Stock-based compensation	$ 9,238	$ 6,310	$ 4,027
Unrecognized stock-based compensation expense	$ 4,500
Remaining weighted-average remaining vesting period	4 months 17 days
Unvested restricted stock awards
Shares
Balance, beginning (in shares)	1,167,984
Vested (in shares)	(573,803)
Repurchased (in shares)	(36,202)
Balance, ending (in shares)	557,979	1,167,984
Weighted Average Grant Date Fair Value Per Share
Balance, beginning (in dollars per share)	$ 0.76
Vested (in dollars per share)	0.77
Repurchased (in dollars per share)	0.67
Balance, ending (in dollars per share)	$ 0.70	$ 0.76
Employees restricted stock
Restricted stock disclosures
Stock-based compensation	$ 500	$ 500	500
Non-employee consultants restricted stock
Restricted stock disclosures
Stock-based compensation	$ 2,700	$ 2,600	$ 2,600

Stock based compensation - Stock Options - (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Options disclosures
Stock-based compensation	$ 9,238	$ 6,310	$ 4,027
Stock options, all inclusive
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding, beginning balance (in shares)	1,871,237
Granted (in shares)	1,720,000
Exercised (in shares)	(158,677)
Cancelled or forfeited (in shares)	(288,994)
Outstanding, ending balance (in shares)	3,143,566	1,871,237
Exercisable (in shares)	955,587
Vested and expected to vest (in shares)	3,143,566
Weighted Average Exercise Price
Outstanding (in dollars per share)	$ 10.21
Granted (in dollars per share)	13.07
Exercised (in dollars per share)	8.59
Cancelled or forfeited (in dollars per share)	10.50
Outstanding (in dollars per share)	11.82	$ 10.21
Exercisable (in dollars per share)	10.45
Vested and expected to vest (in dollars per share)	$ 11.82
Remaining Contractual Life
Outstanding	8 years 7 months 6 days
Exercisable	8 years 1 month 6 days
Vested and expected to vest	8 years 7 months 6 days
Aggregate Intrinsic Value
Outstanding	$ 15,991
Exercisable	5,933
Vested and expected to vest	$ 15,991
Employees and directors stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 8.48
Stock-based compensation	$ 5,500	$ 3,000	700
Non-employees stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 6.88
Stock-based compensation	$ 400	$ 200	$ 300

Stock based compensation - Black-Scholes options pricing model - (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair value assumptions
Risk-free interest rate			1.60%
Expected term			6 years
Expected volatility			78.60%
Options disclosures
Stock-based compensation	$ 9,238	$ 6,310	$ 4,027
Unrecognized stock-based compensation expense	$ 4,500
Remaining weighted-average remaining vesting period	4 months 17 days
Stock options, all inclusive
Options disclosures
Stock options granted (in shares)	1,720,000
Unrecognized stock-based compensation expense	$ 16,100
Remaining weighted-average remaining vesting period	2 years 7 months 28 days
Employees and directors stock options
Fair value assumptions
Risk-free interest rate	2.00%	1.50%
Expected term	6 years	6 years
Expected volatility	73.70%	73.10%
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 8.48
Stock-based compensation	$ 5,500	$ 3,000	$ 700
Non-employees stock options
Fair value assumptions
Risk-free interest rate	2.40%	2.10%	2.00%
Expected term	8 years 6 months	9 years 1 month 6 days	10 years
Expected volatility	76.20%	83.30%	84.00%
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 6.88
Stock-based compensation	$ 400	$ 200	$ 300

401(k) Savings Plan (Details) - USD ($) $ in Millions	12 Months Ended
401(k) Savings Plan (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
401(k) Savings Plan
Employer contribution expense	$ 0.5	$ 0.3

Income taxes (Details) $ in Thousands	12 Months Ended
Income taxes (Details) $ in Thousands	Dec. 31, 2016 USD ($)
Current
Federal	$ 180
Total current	180
Deferred
Federal	(111)
State	(17)
Total deferred	(128)
Total tax expense	$ 52

Income taxes -Components of deferred tax assets and liabilities -(Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income taxes
Tax expense recognized in OCI		$ 128
Income Tax Expense (Benefit)		52
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	$ (235)	199	$ (251)
Deferred tax assets:
Deferred Tax Assets, Operating Loss Carryforwards	24,642	8,927
Tax credit carryforwards	8,832	4,284
Deferred rent	1,458	1,964
Deferred revenue	8,622	16,333
Non‑deductible expenses	817	714
Intangibles	832	998
Stock Compensation	1,361	672
Total deferred tax assets	46,564	33,892
Less valuation allowance	(44,953)	(31,361)
Net deferred tax assets	1,611	2,531
Deferred tax liabilities
Depreciation and amortization	(1,611)	(2,501)
Unrealized gain on available-for-sale securities		30
Net deferred taxes	$ 0	$ 0

Income taxes - Operating loss carryforwards - (Details) - USD ($)		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
Valuation Allowance		$ 44,953,000	$ 31,361,000
Change in the valuation allowance		13,600,000
Unrecognized tax benefits		$ 0	$ 0
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax computed at federal statutory tax rate		34.00%	34.00%	34.00%
State taxes, net of federal benefit		6.10%	5.60%	4.10%
General business credit carryovers		5.00%	4.20%	3.10%
Non‑deductible expenses		(4.10%)	(4.00%)	(15.50%)
Deferred rate change		(21.80%)
Change in valuation allowance		(19.20%)	(40.20%)	(25.70%)
Total			(0.40%)
Forecast
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax computed at federal statutory tax rate	21.00%
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		$ 89,800,000
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		91,700,000
Investment Tax Credit expiring in 2018		300,000
Research and development tax credits carryforwards \| Federal
Operating Loss Carryforwards [Line Items]
Research and development tax credit carryforwards		6,600,000
Research and development tax credits carryforwards \| State
Operating Loss Carryforwards [Line Items]
Research and development tax credit carryforwards		$ 2,300,000

Related party transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investor \| Consulting and management services
Related Party Transaction [Line Items]
Total amount of services received	$ 31.8	$ 0.1	$ 1.3
Genzyme \| Collaboration arrangement
Related Party Transaction [Line Items]
Revenue recognized	10.1	14.2	17.3
Expense recognized	$ 0.1	$ 1.2	$ 2.3

Selected quarterly financial data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 6,346	$ 1,148	$ 1,177	$ 1,464	$ 2,362	$ 3,308	$ 3,720	$ 4,830	$ 10,135	$ 14,220	$ 17,334
Total operating expenses	18,693	24,503	19,816	18,986	16,205	13,679	13,338	12,297	81,998	55,519	37,588
Loss from operations	(12,347)	(23,355)	(18,639)	(17,522)	(13,843)	(10,371)	(9,618)	(7,467)	(71,863)	(41,299)	(20,254)
Net loss attributable to common stockholders	$ (11,828)	$ (23,346)	$ (18,876)	$ (16,648)	$ (14,674)	$ (8,996)	$ (9,335)	$ (7,188)	$ (70,698)	$ (40,193)	$ (38,290)
Net loss per share attributable to common stock, basic and diluted	$ (0.40)	$ (0.89)	$ (0.73)	$ (0.65)	$ (0.57)	$ (0.35)	$ (0.37)	$ (0.29)	$ (2.64)	$ (1.59)	$ (9.14)

Subsequent events - (Details) $ in Millions	1 Months Ended
Subsequent events - (Details) $ in Millions	Feb. 28, 2018 USD ($) ft²
75 Sidney Street
Subsequent Event [Line Items]
Additional space leased \| ft²	11,000
75 Sidney Street \| Subsequent Event
Subsequent Event [Line Items]
Additional space leased \| ft²	11,000
AbbVie \| Collaborative Arrangement \| Subsequent Event
Subsequent Event [Line Items]
Payment received \| $	$ 69.0
Development and regulatory milestone payments \| $	$ 895.0