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Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2015	Dec. 15, 2015
Document and Entity Information
Entity Registrant Name	Voyager Therapeutics, Inc.
Entity Central Index Key	0001640266
Document Type	10-Q
Document Period End Date	Sep. 30, 2015
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		26,749,240
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q3

Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Assets, Current [Abstract]
Cash and cash equivalents	$ 66,533	$ 7,035
Marketable securities	41,138
Prepaid Expense and Other Assets, Current	942	1,323
Total Current Assets	108,613	8,358
Property, Plant and Equipment, Net	3,277	2,804
Deposits and other non-current assets	1,623	335
Marketable securities	52,922
Total Assets	166,435	11,497
Liabilities, Current [Abstract]
Accounts Payable	541	1,554
Accrued expenses	2,942	642
Deferred rent, current portion	294	278
Deferred revenue, current portion	19,589
Total current liabilities	23,366	2,474
Deferred rent, net of current portion	1,093	1,314
Deferred revenue, net of current portion	40,205
Other non-current liabilities	52	255
Preferred stock tranche liability		6,305
Total Liabilities	64,716	10,348
Stockholders' Equity Attributable to Parent [Abstract]
Common stock, $0.001 par value: 95,000,000, and 65,000,000 shares authorized at September 30, 2015 and December 31, 2014 respectively; 1,369,715 and 814,834 shares issued and outstanding at September 30, 2015 and December 31, 2014, respectively	1	1
Accumulated other comprehensive gain	38
Accumulated deficit	(47,178)	(20,831)
Total stockholders’ deficit	(47,139)	(20,830)
Total liabilities, redeemable convertible preferred stock and stockholders’ deficit	166,435	11,497
Series A preferred stock
Liabilities, Current [Abstract]
Preferred stock tranche liability	2,600
Redeemable convertible preferred stock	59,863	$ 21,979
Series B preferred stock
Liabilities, Current [Abstract]
Redeemable convertible preferred stock	$ 88,995

Balance Sheets (Parenthetical) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common Stock, Shares Authorized	95,000,000	65,000,000
Common Stock, Shares, Issued	1,369,715	814,834
Common Stock, Shares, Outstanding	1,369,715	814,834
Series A preferred stock
Redeemable convertible preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Redeemable convertible referred stock, authorized shares	45,000,000	45,000,000
Redeemable convertible preferred stock, issued shares	45,000,000	25,000,000
Redeemable convertible preferred stock, outstanding shares	45,000,000	25,000,000
Redeemable convertible preferred stock, aggregate liquidation preference (in dollars)	$ 48,621	$ 26,086
Series B preferred stock
Redeemable convertible preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Redeemable convertible referred stock, authorized shares	30,000,001	0
Redeemable convertible preferred stock, issued shares	30,000,001	0
Redeemable convertible preferred stock, outstanding shares	30,000,001	0
Redeemable convertible preferred stock, aggregate liquidation preference (in dollars)	$ 93,827	$ 0

Statement of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Statement [Abstract]
Collaboration revenue	$ 4,937		$ 12,397
Operating Expenses [Abstract]
Research and development	6,481	$ 2,399	18,459	$ 5,938
General and administrative	2,475	1,257	6,752	3,933
Total operating expenses	8,956	3,656	25,211	9,871
Operating loss	(4,019)	(3,656)	(12,814)	(9,871)
Other expense, net
Interest income (expense), net	102		175	(2)
Other financing expense		(349)	(9,750)	(1,754)
Total other expense, net	102	(349)	(9,575)	(1,756)
Net loss	(3,917)	(4,005)	(22,389)	(11,627)
Other comprehensive loss
Net unrealized loss on available for-sale securities	58		38
Total other comprehensive loss	58		38
Comprehensive loss	(3,859)	(4,005)	(22,351)	(11,627)
Reconciliation of net loss to net loss attributalbe to common stockholder:
Net loss	(3,917)	(4,005)	(22,389)	(11,627)
Accretion of redeemable convertible preferred stock to redemption value	(2,594)	(406)	(6,045)	(859)
Accrued dividends on Series A redeemable convertible preferred stock	(403)		(1,039)
Net loss attributable to common stockholders	$ (6,914)	$ (4,411)	$ (29,473)	$ (12,486)
Net loss per share attributable to common stock, basic and diluted	$ (5.25)	$ (6.45)	$ (25.36)	$ (21.41)
Weighted-average common shares outstanding, basic and diluted	1,317,150	684,192	1,161,982	583,294

Statement of Cash Flows - USD ($)	9 Months Ended
Statement of Cash Flows - USD ($)	Sep. 30, 2015	Sep. 30, 2014
Cash flow from operating activities
Net loss	$ (22,389,000)	$ (11,627,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	2,061,000	275,000
Depreciation	436,000	73,000
Amortization of premiums and discounts on marketable securities	(261,000)
Change in fair value of preferred stock tranche liability	9,750,000	1,754,000
Non-cash interest on convertible promissory notes payable		2,000
Expense related to shares issued in connection with services performed		250,000
In kind Research And Development Expenses	2,190,000
Deferred rent	205,000	(304,000)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	243,000	(644,000)
Other non-current assets	14,000	(2,000)
Deferred revenue	57,604,000
Accounts payable	(1,013,000)	566,000
Accrued expenses	1,952,000	881,000
Other non-current liabiliites	(186,000)
Lease incentive benefit	138,000	1,042,000
Net cash provided by (used in) operating activities	50,856,000	(7,126,000)
Cash flow from investing activities
Purchases of property and equipment	(909,000)	(1,979,000)
Change in restricted cash		(314,000)
Purchases of marketable securities	(100,283,000)
Proceeds from marketable securities	6,000,000
Net cash used in investing activities	(95,192,000)	(2,293,000)
Cash flow from financing activities
Proceeds from issuance of Series A redeemable convertible preferred stock and tranche rights, net of issuance costs	19,999,000	15,542,000
Proceeds from issuance of Series B redeemable convertible preferred stock, net of discount and issuance costs	84,780,000
Deferred Initial Public Offering Costs	(954,000)
Proceeds from issuance of common stock and restricted stock	9,000	72,000
Net cash provided by financing activities	103,834,000	15,614,000
Net increase in cash and cash equivalents	59,498,000	6,195,000
Cash and cash equivalents, beginning of period	7,035,000	135,000
Cash and cash equivalents, end of period	66,533,000	6,330,000
Supplemental Disclosure of Cash and non-cash Activities
Accretion of redeemable convertible preferred stock to redemption value	6,045,000	859,000
Exchange of promissory notes payable and accrued interest into Series A redeemable convertible preferred stock and tranche rights		$ 2,929,000
Deferred initial public offering costs included in accrued expenses	$ 348,000

Nature of business	9 Months Ended
Nature of business	Sep. 30, 2015
Nature of business
Nature of business	VOYAGER THERAPEUTICS INC. NOTES TO UNAUDITED CONDENSED FINANCIAL STATEMENTS 1. Nature of business Voyager Therapeutics, Inc. (“the Company”) is a clinical‑stage gene therapy company focused on developing life‑changing treatments for patients suffering from severe diseases of the central nervous system (the “CNS”). The Company focuses on CNS diseases where it believes that an adeno‑associated virus (“AAV”) gene therapy approach can have a clinically meaningful impact by either increasing or decreasing the production of a specific protein. The Company has created a product engine that 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 directly to the CNS. The Company’s product engine has rapidly generated programs for five CNS indications, including advanced Parkinson’s disease, a form of monogenic amyotrophic lateral sclerosis, Friedreich’s ataxia, Huntington’s disease and spinal muscular atrophy. The Company’s most advanced clinical candidate, VY‑AADC01, is being evaluated for the treatment of advanced Parkinson’s disease in an open‑label, Phase 1b clinical trial with the goal of generating human proof‑of‑concept data in the second half of 2016. 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 pre‑clinical 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.

Summary of significant accounting policies

9 Months Ended

Sep. 30, 2015

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 unaudited condensed financial statements of the Company have been prepared in accordance with accounting principles generally accepted in the United States (“GAAP”) for interim financial reporting and as required by Regulation S-X, Rule 10-01. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. For further information, refer to the financial statements and footnotes included in the Company’s Registration Statement on Form S-1 for the year ended December 31, 2014 as filed with the Securities and Exchange Commission (“SEC”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”).

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 estimated net proceeds to the Company of $72.7 million after deducting underwriting discounts and commissions and estimated offering expenses payable by the Company. 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 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. In connection with the closing of the IPO, all of the Company’s outstanding redeemable 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. The significant increase in shares outstanding in November 2015 is expected to impact the year-over-year comparability of the Company’s net loss per share calculations over the next year.

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the 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.

Unaudited Interim Financial Information

The accompanying condensed balance sheets as of September 30, 2015, the condensed statements of operations and comprehensive loss for the three and nine months ended September 30, 2015 and 2014, and the condensed statements of cash flows for the nine months ended September 30, 2015 and 2014 are unaudited. The interim unaudited financial statements have been prepared on the same basis as the annual audited financial statements; and in the opinion of management, reflect all adjustments, which include only normal recurring adjustments necessary for the fair statement of the Company’s financial position as of September 30, 2015, the results of its operations and comprehensive loss for the three and nine months ended September 30, 2015 and 2014, and its cash flows for the nine months ended September 30, 2015 and 2014. The financial data and other information disclosed in these notes related to the periods ended September 30, 2015 and 2014 are unaudited. The results for the three and nine months ended September 30, 2015, are not necessarily indicative of results to be expected for the year ending December 31, 2015, any other interim periods, or any future year or period.

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 securities with a remaining maturity when purchased of greater than three months as available‑for‑sale. Marketable securities with a remaining maturity date greater than one year are classified as non‑current where the Company has the intent and ability to hold these securities for at least the next twelve months. Available‑for‑sale securities are maintained by an investment manager and may consist of U.S. Treasury securities and U.S. government agency securities. Available‑for‑sale securities are carried at fair value with the unrealized gains and losses included in other comprehensive income as a component of stockholders’ deficit 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 unrealized loss through a charge to the Company’s statement of operations and comprehensive loss.

There were no marketable securities held as of December 31, 2014.

Marketable securities as of September 30, 2015 consist of the following:


			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value

As of September 30, 2015
Money market funds included in cash and cash equivalents	$	65,371	$	—	$	—	$	65,371
Total money market funds included in cash and cash equivalents	$	65,371	$	—	$	—	$	65,371
Marketable securities:
U.S. Treasury notes		50,152		23		—		50,175
U.S. Government agency bonds		43,870		15		—		43,885
Total marketable securities	$	94,022	$	38	$	—	$	94,060
Total money market funds and marketable securities	$	159,393	$	38	$	—	$	159,431

The estimated fair value of the Company’s marketable securities balance at September 30, 2015, by contractual maturity, is as follows:


Due in one year or less	$	41,138
Due after one year through two years		52,922
Total marketable securities	$	94,060

Restricted Cash

At September 30, 2015 and December 31, 2014, the Company maintained restricted cash totaling approximately $314,000 held in the form of money market accounts as collateral for the Company’s facility lease obligation and credit cards. The balance is included within deposits and other non‑current assets in the accompanying 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 September 30, 2015.

Deferred Issuance Costs

Deferred issuance costs, which primarily consist of direct incremental legal and accounting fees relating to the Company’s proposed initial public offering of common stock are capitalized as incurred. The deferred issuance costs will be offset against proceeds upon the consummation of the offering. Approximately $1,302,000 of deferred issuance costs were incurred and capitalized as of September 30, 2015. No amounts were capitalized as of December 31, 2014. Such costs are included within other non‑current assets on the balance sheet.

Revenue Recognition

As of September 30, 2015, all of the Company’s revenue is generated exclusively from its collaboration agreement with Genzyme Corporation, a Sanofi company (“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 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 judgements 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 judgement. 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 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 measureable 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 of 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 of 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 judgement involved in determining whether a milestone satisfies all of 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 its 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 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 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 of the Company’s common stock, the Company has based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. 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‑based milestone vesting over the remaining service period when management determines that achievement of the milestone is probable. Management evaluates when the achievement of a performance‑based milestone 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, 2014, the Company does not have any significant uncertain tax positions.

Comprehensive Loss

Comprehensive loss is comprised of net loss and other comprehensive loss. Other comprehensive loss consists of unrealized gains and losses on marketable securities for the nine months ended September 30, 2015. There was no other comprehensive loss for the nine months ended September 30, 2014.

Net Loss Per Share

Basic net loss per share of common stock 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 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 calculation, redeemable convertible preferred stock and unvested restricted common stock are considered to be potentially dilutive securities, but are excluded from the calculation of diluted net loss per share because their effect would be anti‑dilutive and therefore, basic and diluted net loss per share 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 because to do so would be anti‑dilutive (in common stock equivalent shares):


	As of September 30,
	2015	2014

Series A redeemable convertible preferred stock	10,588,235	5,882,352
Series B redeemable convertible preferred stock	7,058,819	—
Unvested restricted common stock	2,013,140	2,505,256
Outstanding stock options	852,249	—
Total	20,512,443	8,387,608

The Company’s redeemable convertible preferred stock is entitled to receive dividends based on dividends declared to common stockholders, thereby giving the preferred stockholders the right to participate in undistributed earnings of the Company above the stated dividend rate. However, preferred stockholders do not have a contractual obligation to share in the losses of the Company. As of December 31, 2014 and all prior periods reported, the Company has been in a net loss position; therefore, the Company’s accounting for basic and diluted earnings per share was unaffected by the participation rights of the preferred stockholders. 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.

The following table summarizes the Company’s unaudited pro forma net loss per share attributable to common stockholders (in thousands, except share and per share data):



	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2015		2014		2015		2014

Pro forma net loss attributable to common stockholders	$	(6,914)	$	(4,411)	$	(29,473)	$	(12,486)
Weighted average number of common shares outstanding, basic and diluted		1,317,150		684,192		1,161,982		583,294
Add:
Dilutive effect of common share equivalents resulting from common share options and preferred common shares (as converted)		—		—		—		—
		1,317,150		684,192		1,161,982		583,294
Pro forma basic and diluted net loss per share attributable to common stockholders	$	(5.25)	$	(6.45)	$	(25.36)	$	(21.41)

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 and expects to continue to rely 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. ASU 2014‑09 amends Accounting Standards Codification ASC 605, Revenue Recognition, by outlining a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers. ASU 2014‑09 will be effective for the Company for interim and annual periods beginning after December 15, 2017. The Company is evaluating the impact that this ASU may have on its financial statements, if any.

In August 2014, the FASB issued ASU No. 2014‑15, which requires management to assess an entity’s ability to continue as a going concern every reporting period, and provide certain disclosures if management has substantial doubt about the entity’s ability to operate as a going concern, or an express statement if not, by incorporating and expanding upon certain principles that are currently in U.S. auditing standards. This guidance is effective for the annual period ending after December 15, 2016, and for annual periods and interim periods thereafter. Early application is permitted. The Company is in process of evaluating this guidance and determining the expected effect on its financial statements.

In April 2015, the FASB issued ASU No. 2015-03, which amends existing guidance to require the presentation of debt issuance costs in the balance sheet as a deduction from the carrying amount of the related debt liability instead of a deferred charge. ASU No. 2015-03 is effective for annual reporting periods beginning after December 15, 2015, and early adoption is permitted. The amendment must be applied retrospectively such that the balance sheet of each individual period presented is adjusted to reflect the period-specific impact of using the new guidance. Upon transition, a business must adhere to the appropriate disclosures for an adjustment in an accounting principle. Such disclosures include why the change in accounting principle is occurring, the transition method, an explanation of the prior period information that was retrospectively adjusted, and how the change impacts the financial statement line items (i.e., debt issuance cost asset and the debt liability). The Company is currently in the process of evaluating the timing and impact of adoption.

Fair Value Measurements

9 Months Ended

Sep. 30, 2015

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, 2014 are as follows (in thousands):


			Quoted Prices		Significant
			in Active		Other		Significant
			Markets for		Observable		Unobservable
	December 31,		Identical Assets		Inputs		Inputs
Liabilities	2014		(Level 1)		(Level 2)		(Level 3)
Convertible preferred stock tranche liability	$	6,305	$	—	$	—	$	6,305
Total	$	6,305	$	—	$	—	$	6,305

The Company estimates the fair value of its Series A redeemable convertible preferred stock (“Series A Preferred Stock”) tranche liability at the time of issuance and subsequently remeasures it using a probability‑weighted present value model that considers the probability of closing a tranche (67%), the estimated future value of Series A Preferred Stock at closing ($1.51), and the investment required ($20.0 million) at closing. Future values are converted to present value using a discount rate (16.2%) appropriate for probability‑adjusted cash flows. These estimates are based, in part, on subjective assumptions. Changes to these assumptions as well as the Company’s stock value on the reporting date can have a significant impact on the fair value of the Series A Preferred Stock tranche liability.

The following table provides a reconciliation of all assets and liabilities measured at fair value using Level 3 significant unobservable inputs (in thousands):


	Preferred Stock
	Tranche Liability
Balance at December 31, 2014	$	6,305
Changes in fair value		9,750
Reclassification to Series A Preferred Stock		(16,055)
Balance at September 30, 2015	$	—

Assets and liabilities measured at fair value on a recurring basis as of September 30, 2015 are as follows (in thousands):


			Quoted Prices		Significant
			in Active		Other		Significant
			Markets for		Observable		Unobservable
	September 30,		Identical Assets		Inputs		Inputs
Assets	2015		(Level 1)		(Level 2)		(Level 3)

Money market funds included in cash and cash equivalents	$	65,371	$	65,371	$	—	$	—
Marketable securities:
U.S. Treasury notes		50,175		50,152		—		—
U.S. government agency securities		43,885		—		43,870		—
Total	$	159,431	$	115,523	$	43,870	$	—

The Company measures the fair value of money market funds and U.S. Treasuries based on quoted prices in active markets for identical securities. The Level 2 marketable securities include U.S. government and 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 Company did not hold any marketable securities at December 31, 2014.

- 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

9 Months Ended

Sep. 30, 2015

Prepaid expense and other current assets

Prepaid expenses and other current assets

4. Prepaid Expenses and Other Current Assets

Prepaid expense and other current assets consist of the following (in thousands):


	As of			As of
	September 30,			December 31,
	2015		2014

Prepaid expenses	$	679	$	900
Other current assets		263		423
Total	$	942	$	1,323

Property and equipment, net

9 Months Ended

Sep. 30, 2015

Property and equipment, net

Property and equipment, net

5. Property and Equipment, net

Property and equipment, net consists of the following (in thousands):


	As of			As of
	September 30,			December 31,
	2015		2014

Laboratory equipment	$	2,081	$	1,223
Furniture and office equipment		484		441
Leasehold improvements		1,332		1,324
		3,897		2,988
Less: accumulated depreciation		(620)		(184)
Property and equipment, net	$	3,277	$	2,804

The Company recorded $63,000 and $160,000 during the three months ended September 30, 2015 and September 30, 2014, respectively. The Company recorded $73,000 and $436,000 in depreciation expense during the nine months ended September 30, 2014 and September 30, 2015, respectively.

Accrued expenses

9 Months Ended

Sep. 30, 2015

Accrued expenses

Accrued expenses

6. Accrued Expenses

Accrued expenses consist of the following (in thousands):


	As of			As of
	September 30,			December 31,
	2015		2014

Patent costs	$	250	$	274
Research and development costs		1,460		125
Professional services		570		81
Employee compensation costs		176		85
Other		486		77
Total	$	2,942	$	642

Commitments and contingencies

9 Months Ended

Sep. 30, 2015

Commitments and Contingencies

Commitments and contingencies

7. Commitments and Contingencies

Operating Leases

During March 2014, the Company entered into an agreement to lease its facility under a non‑cancelable operating lease that expires 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.

The Company received a leasehold improvement incentive from the landlord totaling $1,250,000. The Company recorded these incentives as a component of deferred rent and will amortize these incentives as a reduction of rent expense over the life of the lease. These leasehold improvements have been recorded as fixed assets.

Rent expense of approximately $168,000 and $140,000 and $403,000 and $590,000 was incurred during the three and nine months ended September 30, 2014 and September 30, 2015, respectively.

Future annual minimum lease payments at December 31, 2014 are as follows (in thousands):


	Total Minimum
	Lease Payments
2015	$	1,117
2016		1,170
2017		1,192
2018		1,214
2019		1,184
	$	5,877

Significant Agreements

Genzyme Collaboration Agreement

Summary of Agreement

In February 2015, the Company entered into an agreement with Genzyme (“Collaboration Agreement”), which included a non‑refundable upfront payment of $65.0 million. In addition, contemporaneous with entering into the Collaboration Agreement, Genzyme entered into a Series B Stock Purchase Agreement, under which 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 Collaboration Agreement.

Under the Collaboration Agreement, the Company granted 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‑AADC01 (“Parkinson’s Program”), VY‑FXN01 (“Friedreich’s Ataxia Program”), a future program to be designated by 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”). 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 Genzyme, the Company will collaborate with Genzyme in the development of products under each Split Territory Program and VY‑SMN101 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 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 Genzyme technology relating to the VY‑AADC01 Program, the VY‑HTT01 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 Genzyme with the exception of the following: (i) at the Company’s request and upon mutual agreement, Genzyme will provide “in‑kind” services valued at up to $5.0 million and (ii) 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 Genzyme is entitled to receive from a patient advocacy group, collectively Genzyme “in‑kind” and other funding.

Other than the Parkinson’s Program (for which a POP Study has already been commenced), 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 Genzyme), and Genzyme has not terminated the Collaboration Agreement with respect to the collaboration program, then 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 Genzyme under the 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, 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 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 Genzyme shall have sole responsibility for development of such Split Territory Licensed Product in the rest of the world. The Company and 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. 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 Genzyme exercised its co‑commercialization rights, 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, Genzyme shall have the sole right to develop the Spinal Muscular Atrophy Product worldwide. Genzyme shall be responsible for all of the development costs that occur after the option exercise date for the Spinal Muscular Atrophy Program. Genzyme is also responsible for commercialization activities relating to the Spinal Muscular Atrophy Product worldwide.

Genzyme is required to pay the Company for specified regulatory and commercial milestones, if achieved, up to $645 million across all programs. 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 $265 million. The commercial milestones are payable upon achievement of specified annual net sales in each program and range from $50.0 million to $100 million per milestone, with an aggregate total of $380 million.

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 Genzyme. 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 Genzyme technology in the Split Territory Licensed Product. If Genzyme elects to co‑commercialize VY‑HTT01 in the United States, the Company and Genzyme will share in any profits or losses from VY‑HTT01 product sales.

The 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 Genzyme. The Company and Genzyme have customary termination rights including the right to terminate for an uncured material breach of the agreement committed by the other party and Genzyme has the right to terminate for convenience.

Accounting Analysis

The 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 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 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 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 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 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 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 best estimate of selling price (“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 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 amounts allocated to the license option 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 at the time of exercise.

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 nine month period ended September 30, 2015, the Company recognized $12,397,000 of revenue associated with its collaboration with Genzyme related to research and development services performed during the period. As of September 30, 2015, there is $59,794,000 of deferred revenue related to the 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 Genzyme has the option to license under the Collaboration Agreement consist of internal and external research and development costs, which primarily include: salaries and benefits, lab supplies and preclinical research studies. The Company does not separately track or segregate the amount of costs incurred under the Collaboration Agreement. All of these costs are included in research and development expenses in the Company’s statement of operations during the nine months ended September 30, 2015. The Company estimates that the majority of research and development expense during the period relate to programs for which Genzyme has an option right.

University of Massachusetts (“UMass”) and MassBiologics Collaboration

In January 2014, UMass and the Company entered into a Collaboration Agreement wherein the Company granted UMass 23,529 shares of common stock, valued at $12,000, which was recorded as research and development expense. This was the only payment made under the Collaboration Agreement until it was amended by the Collaboration Agreement entered into with UMass and MassBiologics in October 2014.

On October 20, 2014, the Company entered into a Collaboration Agreement with UMass and MassBiologics (of the UMass Medical School).

Under the agreement, the Company shall (i) fund certain projects that will be conducted by UMass or MassBiologics, (ii) fund certain educational programs of UMass, including post‑doctoral research at the Company’s laboratories beginning in 2015, and (iii) collaborate with MassBiologics to establish scalable processes for manufacturing recombinant adeno‑associated viral (“rAAV”) vector products using current good manufacturing practices.

In November 2014, the parties agreed to the first project under this agreement whereby the Company will fund approximately $2,861,000 over a sixteen month period for certain research and development services performed by MassBiologics. The project commenced in January 2015. If the agreement is terminated for any reason, the Company is obligated to fund the remaining balance of the total price of all work completed and any other out of pocket costs incurred by MassBiologics on behalf of the Company. As of December 31, 2014 and September 30, 2015, the Company had provided cumulative funding of approximately $376,000 and $1,058,000, respectively, which exceeded costs incurred by $376,000 at December 31, 2014 and approximately $701,000 funding was due at September 30, 2015. The amount funded in excess of costs incurred is recorded in prepaid expenses as of the balance sheet at December 31, 2014 and recorded in accrued expenses for the amounts due at September 30, 2015. Research and development costs incurred by MassBiologics under the project agreement will be expensed by the Company as incurred.

Other Agreements

During 2014 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 year ended December 31, 2014 and nine months ended September 30, 2014 and 2015, the Company paid $830,000, $800,000 and $75,000, respectively, in up‑front 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 $12.0 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 up‑front fees of up to approximately $1.5 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, 2014 and September 30, 2015, 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, 2014 and the nine months ended September 30, 2014 and September 30, 2015, the Company incurred $839,000, $740,000 and $131,000 of expense, respectively, related to these reimbursable patent costs which are recorded as general and administrative expense.

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, 2014 or September 30, 2015.

Redeemable Convertible Preferred Stock

9 Months Ended

Sep. 30, 2015

Redeemable Convertible Preferred Stock

Redeemable Convertible Preferred Stock [Texl Block]

8. Redeemable Convertible Preferred Stock

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.

Series A Preferred Stock

At September 30, 2015, 45,000,000 shares of Series A Preferred Stock were authorized, issued and outstanding. 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 $42,039,000, net of issuance costs of $32,000, and the exchange of outstanding redeemable Convertible Notes, including accrued interest, of approximately $2,929,000. The Series A Preferred Stock have a liquidation preference amount of $48,621,000 at September 30, 2015.

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 are obligated to purchase and the Company is 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 are legally detachable and separately exercisable from the Series A Preferred Stock. Therefore, the Company has allocated the proceeds between the Tranche Rights and the Series A Preferred Stock. As the Series A Preferred Stock is redeemable at the holder’s option, the Tranche Rights are classified as an asset or liability and are initially recorded at fair value. The Tranche Rights are measured at fair value at each reporting period. Since the Tranche Rights are 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 are converted to present value using a discount rate appropriate for probability‑adjusted cash flows.

The following table summarizes the initial value of the Tranche Rights included in the Series A Preferred Stock Purchase Agreement (in thousands):


	Fair Value of
	Tranche Right
	Asset (Liability)
Tranche Right I	$	1,495
Tranche Right II		(4,095)
Total value of Tranche Rights	$	(2,600)

Tranche Right I was initially recorded as an asset of $1,495,000 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 $18,491,000, 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. The Company recognized income of $261,000 related to the mark to market of Tranche Right I during the year ended December 31, 2014, which is included in other financing expense.

Tranche Right II was initially recorded as a liability of $4,095,000 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. The Company recognized expense of $2,210,000 related to the mark to market of Tranche Right II during the year ended December 31, 2014, which is included in other financing expense.

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 $9,750,000 related to the mark to market of Tranche Right II during the period ended September 30, 2015, which is 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 $36,054,000 which exceeds the redemption value, therefore the carrying value is not being subsequently adjusted. However, the Company has reflected accrued dividends of approximately $1,039,000 related to this issuance in the net loss attributable to common shareholders for the nine months ended September 30, 2015.

Series B Preferred Stock

At September 30, 2015, 30,000,001 shares of Series B Preferred Stock were authorized, issued and outstanding. These shares were issued for $3.00 per share. This issuance resulted in cash proceeds of $89,780,000, net of issuance costs of $220,000. Additionally, a discount of $5,000,000 was recorded against the proceeds as the amount paid by Genzyme was in excess of fair value of the Series B Preferred Stock at issuance. The Series B Preferred Stock has a liquidation preference amount of $93,827,000.

Preferred Stock

The rights, preferences, and privileges of the Preferred Stock are listed below:

Conversion

Shares of Preferred Stock are convertible at any time at the option of the holder into such number of shares as is determined by dividing the original issuance price by the conversion price in effect at the time. As of September 30, 2015, the conversion price was $4.25 for Series A Preferred Stock and $12.75 for Series B Preferred Stock, subject to adjustments to reflect the issuance of Common Stock, options, warrants, or other rights to subscribe for or to purchase Common Stock for a consideration per share, less than the conversion price then in effect and subsequent stock dividends and stock splits. In addition any reorganization, recapitalization, reclassification, consolidation or merger in which common stock is exchanged for securities, cash or other property.

All outstanding shares of Preferred Stock are automatically converted upon the completion of either an IPO resulting in gross proceeds to the Company of at least $50.0 million or the vote or written consent of 67% of the then outstanding shares of preferred stock.

Dividends

Holders of Preferred Stock are entitled to receive, before any cash is paid out or set aside for any Common Stock, cash dividends at a rate of 8% of the original purchase price per share annually (the “Accruing Dividends”). The dividends accrue cumulatively on a daily basis and are payable only when, and if, declared by the Board of Directors or upon liquidation or redemption.

In addition, the holders of Preferred Stock are entitled to additional dividends based on dividends declared to common stockholders, thereby giving the preferred stockholders the right to participate in undistributed earnings of the Company above the stated per share dividend rate. The preferred stockholders do not have a contractual obligation to share in the losses of the Company

No dividends have been declared since the Company’s inception. Aggregate cumulative preferred dividends on Preferred Stock at September 30, 2015 were $7,448,000.

Redemption

The Preferred Stock is redeemable at the option of the holder after the redemption date of February 2021. The redemption value of the Preferred Stock is equal to $3.00 per share for Series B Preferred Stock and $1.00 per share for Series A Preferred Stock plus any accrued but unpaid dividends. Accordingly, the Preferred Stock is being accreted to redemption value through its redemption date, including accruals for cumulative dividend rights. If the initial carrying value exceeds the redemption value the carrying value is not adjusted.

Liquidation Preference

Holders of Series B Preferred Stock and Series A Preferred Stock have preference to the assets of the Company in the event of any voluntary or involuntary liquidation, dissolution or winding‑up of the Company, equal to $3.00 per share for Series B Preferred Stock and $1.00 per share for Series A Preferred Stock, plus any accrued but unpaid dividends, whether or not declared, plus any dividends declared but unpaid thereon, on a pari passu basis. After the payment of the preference amounts to the holders of Series B Preferred Stock and Series A Preferred Stock, the remaining assets of the Company are to be distributed among the holders of Series A Preferred Stock and holders of Common Stock on a pro rata basis. However, if the aggregate amount which the holders of Series A Preferred Stock would be entitled to receive exceeds $2.50 per share (subject to appropriate adjustment in the event of any stock dividend, stock split, combination, reclassification or other similar event) (the “Maximum Participant Amount”), each holder of Series A Preferred Stock will receive the greater of the Maximum Participant Amount or the amount such holder would have received if all shares of Series A Preferred Stock had been converted into Common Stock immediately prior to such liquidation.

Voting Rights

Holders of Series A Preferred Stock and Series B Preferred Stock are entitled to vote as a single class with the holders of Common Stock on all matters submitted for vote to the Stockholders of the Company. The holders of Preferred Stock are entitled to one vote for each equivalent common share on an as‑converted basis. In addition, the holders of Series A Preferred Stock are entitled to elect two (2) directors. The remaining directors shall be elected by the holders of Common Stock voting together with the holders of the Series B Preferred Stock as one class on an as‑converted basis.

The holders of Series A Preferred Stock and Series B Preferred Stock have certain protective rights as defined. These protective rights require the Required Vote before action can be taken to (i) increase or decrease the number of shares of Series A Preferred Stock or Series B Preferred Stock that the Company has authority to issue, (ii) change the par value of the Series A Preferred Stock or Series B Preferred Stock, (iii) amend the Certificate of Incorporation in any way that adversely affects the holders of the Series A Preferred Stock or Series B Preferred Stock.

Common Stock

9 Months Ended

Sep. 30, 2015

Common stock

Stockholders' Equity Note Disclosure [Text Block]

9. Common Stock

As of December 31, 2014, and September 30, 2015, the Company had authorized 65,000,000 and 95,000,000 shares of Common Stock, respectively 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. Cash dividends may not be declared or paid to holders of shares of Common Stock until all accrued unpaid dividends on Series A Preferred Stock and Series B Preferred Stock have been paid in accordance with their terms. No dividends have been declared or paid by the Company since its inception.

Liquidation

After payment to of their respective liquidation preferences to the holders of shares of Series A Preferred Stock and Series B Preferred Stock, 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	As of December 31,
	September 30, 2015	2014

Shares reserved for Series A Preferred Stock outstanding	10,588,235	5,882,352
Shares reserved for future issuances of Series A Preferred Stock	—	4,705,883
Shares reserved for Series B Preferred Stock outstanding	7,058,819	—
Shares reserved for vesting of restricted stock awards under the Founder Agreements	889,715	1,068,383
Shares reserved for vesting of restricted stock awards under the 2014 Option and Stock Plan	1,123,425	1,510,434
Shares reserved for exercise of stock options	852,249	—
Shares reserved for issuances under the 2014 Stock Option and Grant Plan	92,534	291,052
	20,604,977	13,458,104

- Definition

The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income.

+ References

Reference 1: 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),(e))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 2: 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=6928386&loc=d3e21484-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SAB TOPIC 4.E)
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21488-112644

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.29-31)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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-Subparagraph (SAB TOPIC 4.C)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187143-122770

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-Topic 505
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-Paragraph 6
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Reference 8: 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 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6405834&loc=d3e23285-112656

Reference 10: 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=6928386&loc=d3e21475-112644

Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Preferred Stock
-URI http://asc.fasb.org/extlink&oid=6521494

Reference 12: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21564-112644

Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 4
-Section E

Reference 15: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 4
-Section C

+ Details

Stock-based compensation

9 Months Ended

Sep. 30, 2015

Stock based compensation

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

10. Stock‑Based Compensation

2014 Stock Option and Grant Plan

In January 2014 the Company adopted the Voyager Therapeutics, Inc. 2014 Stock Option and Grant Plan (the “2014 Plan”), under which it may 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 and August 2015 the Company further amended the Plan to allow for the issuance of up to 2,000,000, 2,047,058 and 2,669,411 shares of Common Stock, respectively. During the year ended December 31, 2014, the Company issued only restricted stock awards under the Plan and during the nine months ended September 30, 2015 the Company only granted stock options.

The terms of stock awards agreements, including vesting requirements, are determined by the Board of Directors and are 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.

During the year ended December 31, 2014, the Company granted a total of 1,597,988 shares of restricted stock to employees and 110,960 shares of restricted stock to non‑employee consultants at an original issuance price of $0.04 per share. During the nine months ended September 30, 2015 the Company issued 794,309 stock options to employees and directors and 60,382 stock options to non‑employees. As of September 30, 2015, there were 92,534 shares available for future issuance under the 2014 Plan.

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. As of December 31, 2014 and September 30, 2015, management has concluded that achievement of such performance‑based milestones was not probable. Accordingly, no stock‑based compensation expense was recorded as of December 31, 2014 and September 30, 2015 related to these awards.

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:


		Three Months Ended			Nine Months Ended
		September 30,			September 30,
		2014		2015	2014		2015

Research and development	$	134	$	627	$	212	$	1,486
General and administrative		34		230		63		575
Total stock-compensation expense	$	168	$	857	$	275	$	2,061

Restricted Stock

A summary of the status of and changes in unvested restricted stock as of December 31, 2014 and September 30, 2015 was as follows:


		Weighted
		Average
		Grant Date
		Fair Value
	Shares	Per Share
Unvested restricted common stock as of December 31, 2014	2,578,817	$	0.77
Issued	—
Vested	(553,605)	$	0.73
Repurchased	(12,072)	$	0.86
Unvested restricted common stock as of September 30, 2015	2,013,140	$	0.77

The expense related to awards granted to employees and non‑employees was $134,000 and $457,000, respectively, for the three months ended September 30, 2015. The expense related to awards granted to employees and non‑employees was $325,000 and $1,119,000, respectively, for the nine months ended September 30, 2015.

As of September 30, 2015, the Company had unrecognized stock‑based compensation expense related to its unvested restricted stock awards of $5,064,000, which is expected to be recognized over the remaining weighted average vesting period of 2.39 years.

The aggregate fair value of restricted stock awards vested during the year ended December 31, 2014 and the nine months ended September 30, 2015, based on estimated fair values of the stock underlying the restricted stock awards on the day of vesting, was $253,000 and $2,658,000 respectively.

Stock Options

A summary of the status of, and changes in, stock options as of December 31, 2014 and September 30, 2015 was as follows:


		Weighted		Remaining	Aggregate
		Average		Contractual	Intrinsic
		Exercise		Life	Value
	Shares	Price		(in years)	(in thousands)
Outstanding at December 31, 2014	—
Granted	854,691	$	7.91
Exercised	(1,276)	$	7.27
Cancelled or forfeited	(1,166)	$	7.27
Outstanding at September 30, 2015	852,249	$	7.91	9.8	$	1,338
Exercisable at September 30, 2015	71,238	$	7.35	9.6	$	151
Vested and expected to vest at September 30, 2015	852,249	$	7.91	9.8	$	1,338

Using the Black‑Scholes option pricing model, the weighted average fair value of options granted to employees and directors during the three and nine months ended September 30, 2015 was $5.66 and $5.39, respectively, and the weighted average fair value to non-employees was $6.90 and $6.23, respectively. The expense related to awards granted to employees and directors was $215,000 and $352,000, for the three and nine months ended September 30, 2015, respectively. There were no stock options granted during the period and year ended December 31, 2014.

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:



	Three Months Ended		Nine Months Ended
	September 30, 2015		September 30, 2015
Risk-free interest rate	1.7	%	1.6	%
Expected dividend yield	—		—
Expected term (in years)	6.0		6.0
Expected volatility	78.4	%	78.6	%

Using the Black‑Scholes option pricing model, the weighted average fair value of options granted to non‑employees during the nine months ended September 30, 2015 was $6.23. Unvested options granted to non‑employees are revalued at each measurement period until they vest. The expense related to awards granted to non‑employees was $49,000 and $263,000 for the three and nine months ended September 30, 2015, respectively. There were no stock options granted during the period and year ended December 31, 2014.

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
	September 30, 2015
Risk-free interest rate	2.2	%
Expected dividend yield	—
Expected term (in years)	10.0
Expected volatility	82.6	%

As of September 30, 2015, the Company had unrecognized stock‑based compensation expense related to its unvested stock options of $4,084,000 which is expected to be recognized over the remaining weighted average vesting period of 3.55 years.

401(k) Savings Plan	9 Months Ended
401(k) Savings Plan	Sep. 30, 2015
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. As currently established, the Company is not required to make and has not made any contributions to the 401(k) Plan to date.

Related party transactions	9 Months Ended
Related party transactions	Sep. 30, 2015
Related party transactions
Related party transactions	12. Related‑Party Transactions Since inception, the Company received consulting and management services from one of its investors. In January 2014, the Company issued 470,589 shares of common stock as partial compensation for these services. The fair value of the shares was approximately $238,000. The total amount of consulting and management services provided by this investor was approximately $117,000 and $6,000, $1,063,000 and $115,000 during the three and nine months ended September 30, 2014 and 2015, respectively. As of September 30, 2015, the Company included approximately $6,000, in accounts payable related to service fees charged by this investor. During the nine month period ended September 30, 2015 the Company recognized $12,397,000 of revenue associated with its collaboration with Genzyme related to research and development services provided during this period. The company also recognized $2.2 million of expense during the nine months ended September 30, 2015 related to in‑kind services provided by Genzyme associated with the collaboration arrangement.

- Definition

The entire disclosure for related party transactions. Examples of related party transactions include transactions between (a) a parent company and its subsidiary; (b) subsidiaries of a common parent; (c) and entity and its principal owners; and (d) affiliates.

+ References + Details

Subsequent events	9 Months Ended
Subsequent events	Sep. 30, 2015
Subsequent events
Subsequent events	13. Subsequent Events 2014 Stock Option and Grant Plan In October 2015, the Company amended the 2014 Plan to increase the number of shares reserved for issuance to 2,998,823 shares. 2015 Stock Option Plan In October 2015, the Company’s board of directors and stockholders approved the 2015 Stock Option and Incentive Plan, or 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 our 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. Any options or awards outstanding under the 2014 Stock Option 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. 2015 Employee Stock Purchase Plan In October 2015 the Company’s board of directors and stockholders approved the 2015 Employee Stock Purchase Plan. A total of 262,362 shares of common stock were initially authorized for issuance under this plan. The 2015 Employee Stock Purchase Plan became effective upon the completion of the IPO. Initial Public Offering On November 16, 2015 the Company completed the sale of 5,750,000 shares of its common stock in its IPO, at a price to the public of $14.00 per share, resulting in net proceeds to the Company of $72.7 million after deducting underwriting discounts and commissions and estimated offering expenses payable by the Company. 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 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. In connection with the closing of the IPO, all of the Company’s outstanding redeemable 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.

Summary of significant accounting policies (Policies)

9 Months Ended

Sep. 30, 2015

Summary of significant accounting policies

Basis of presentation

Use of estimates

Unaudited interim financial information

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

Restricted cash

Property and equipement

Impairment of long-lived assets

Deferred Issuance Costs

Revenue Recognition

As of September 30, 2015, all of the Company’s revenue is generated exclusively from its collaboration agreement with Genzyme Corporation, a Sanofi company (“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

Net loss per share

Concentration of credit risk and off-balance sheet risk

Concentration of suppliers

Segment information

Recent accounting pronouncements

Summary of significant accounting policies (Tables)

9 Months Ended

Sep. 30, 2015

Summary of significant accounting policies

Schedule of Marketable securities

Marketable securities as of September 30, 2015 consist of the following:


			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value

As of September 30, 2015
Money market funds included in cash and cash equivalents	$	65,371	$	—	$	—	$	65,371
Total money market funds included in cash and cash equivalents	$	65,371	$	—	$	—	$	65,371
Marketable securities:
U.S. Treasury notes		50,152		23		—		50,175
U.S. Government agency bonds		43,870		15		—		43,885
Total marketable securities	$	94,022	$	38	$	—	$	94,060
Total money market funds and marketable securities	$	159,393	$	38	$	—	$	159,431

Estimated fair value of marketable securities by contractual maturity

The estimated fair value of the Company’s marketable securities balance at September 30, 2015, by contractual maturity, is as follows:


Due in one year or less	$	41,138
Due after one year through two years		52,922
Total marketable securities	$	94,060

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


	As of September 30,
	2015	2014

Series A redeemable convertible preferred stock	10,588,235	5,882,352
Series B redeemable convertible preferred stock	7,058,819	—
Unvested restricted common stock	2,013,140	2,505,256
Outstanding stock options	852,249	—
Total	20,512,443	8,387,608

Pro forma net loss per share attributable to common stockholders

The following table summarizes the Company’s unaudited pro forma net loss per share attributable to common stockholders (in thousands, except share and per share data):



	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2015		2014		2015		2014

Pro forma net loss attributable to common stockholders	$	(6,914)	$	(4,411)	$	(29,473)	$	(12,486)
Weighted average number of common shares outstanding, basic and diluted		1,317,150		684,192		1,161,982		583,294
Add:
Dilutive effect of common share equivalents resulting from common share options and preferred common shares (as converted)		—		—		—		—
		1,317,150		684,192		1,161,982		583,294
Pro forma basic and diluted net loss per share attributable to common stockholders	$	(5.25)	$	(6.45)	$	(25.36)	$	(21.41)

Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2015

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, 2014 are as follows (in thousands):


			Quoted Prices		Significant
			in Active		Other		Significant
			Markets for		Observable		Unobservable
	December 31,		Identical Assets		Inputs		Inputs
Liabilities	2014		(Level 1)		(Level 2)		(Level 3)
Convertible preferred stock tranche liability	$	6,305	$	—	$	—	$	6,305
Total	$	6,305	$	—	$	—	$	6,305

Assets and liabilities measured at fair value on a recurring basis as of September 30, 2015 are as follows (in thousands):


			Quoted Prices		Significant
			in Active		Other		Significant
			Markets for		Observable		Unobservable
	September 30,		Identical Assets		Inputs		Inputs
Assets	2015		(Level 1)		(Level 2)		(Level 3)
	(unaudited)
Money market funds included in cash and cash equivalents	$	65,371	$	65,371	$	—	$	—
Marketable securities:
U.S. Treasury notes		50,175		50,152		—		—
U.S. government agency securities		43,885		—		43,870		—
Total	$	159,431	$	115,523	$	43,870	$	—

Assets and liabilities measured at fair value using Level 3 significant unobservable inputs

The following table provides a reconciliation of all assets and liabilities measured at fair value using Level 3 significant unobservable inputs (in thousands):


	Preferred Stock
	Tranche Liability
Balance at December 31, 2014	$	6,305
Changes in fair value		9,750
Reclassification to Series A Preferred Stock		(16,055)
Balance at September 30, 2015	$	—

Prepaid expenses and other current assets (Tables)

9 Months Ended

Sep. 30, 2015

Prepaid expense and other current assets

Schedule of prepaid expense and other current assets

4. Prepaid Expenses and Other Current Assets

Prepaid expense and other current assets consist of the following (in thousands):


	As of			As of
	September 30,			December 31,
	2015		2014

Prepaid expenses	$	679	$	900
Other current assets		263		423
Total	$	942	$	1,323

Property and equipment, net (Tables)

9 Months Ended

Sep. 30, 2015

Property and equipment, net

Schedule of property and equipment, net

Property and equipment, net consists of the following (in thousands):


	As of			As of
	September 30,			December 31,
	2015		2014

Laboratory equipment	$	2,081	$	1,223
Furniture and office equipment		484		441
Leasehold improvements		1,332		1,324
		3,897		2,988
Less: accumulated depreciation		(620)		(184)
Property and equipment, net	$	3,277	$	2,804

Accrued expenses (Tables)

9 Months Ended

Sep. 30, 2015

Accrued expenses

Schedule of accrued expenses

Accrued expenses consist of the following (in thousands):


	As of			As of
	September 30,			December 31,
	2015		2014

Patent costs	$	250	$	274
Research and development costs		1,460		125
Professional services		570		81
Employee compensation costs		176		85
Other		486		77
Total	$	2,942	$	642

Commitments and contingencies (Tables)

9 Months Ended

Sep. 30, 2015

Commitments and Contingencies

Future annual minimum lease payments

Future annual minimum lease payments at December 31, 2014 are as follows (in thousands):


	Total Minimum
	Lease Payments
2015	$	1,117
2016		1,170
2017		1,192
2018		1,214
2019		1,184
	$	5,877

Allocable arrangement considateraition 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

Redeemable Convertible Preferred Stock (Tables)

9 Months Ended

Sep. 30, 2015

Redeemable Convertible Preferred Stock

Summary of initial value of Tranche Rights included in Series A Preferred Stock Purchase Agreement

The following table summarizes the initial value of the Tranche Rights included in the Series A Preferred Stock Purchase Agreement (in thousands):


	Fair Value of
	Tranche Right
	Asset (Liability)
Tranche Right I	$	1,495
Tranche Right II		(4,095)
Total value of Tranche Rights	$	(2,600)

- Definition

Tabular disclosure of pertinent information about a derivative or group of derivatives on a disaggregated basis, such as for individual instruments, or small groups of similar instruments. May include a combination of the type of instrument, risks being hedged, notional amount, hedge designation, related hedged item, inception date, maturity date, or other relevant item.

+ References

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+ Details

Common Stock (Tables)

9 Months Ended

Sep. 30, 2015

Common stock

Schedule of shares reserved for future issuance

Shares Reserved For Future Issuance


	As of	As of December 31,
	September 30, 2015	2014

Shares reserved for Series A Preferred Stock outstanding	10,588,235	5,882,352
Shares reserved for future issuances of Series A Preferred Stock	—	4,705,883
Shares reserved for Series B Preferred Stock outstanding	7,058,819	—
Shares reserved for vesting of restricted stock awards under the Founder Agreements	889,715	1,068,383
Shares reserved for vesting of restricted stock awards under the 2014 Option and Stock Plan	1,123,425	1,510,434
Shares reserved for exercise of stock options	852,249	—
Shares reserved for issuances under the 2014 Stock Option and Grant Plan	92,534	291,052
	20,604,977	13,458,104

- 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

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-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 30
-Article 5

+ Details

Stock based compensation (Tables)

9 Months Ended

Sep. 30, 2015

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

Compensation cost recognized for all stock-based compensation awards


		Three Months Ended			Nine Months Ended
		September 30,			September 30,
		2014		2015	2014		2015

Research and development	$	134	$	627	$	212	$	1,486
General and administrative		34		230		63		575
Total stock-compensation expense	$	168	$	857	$	275	$	2,061

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, 2014	2,578,817	$	0.77
Issued	—
Vested	(553,605)	$	0.73
Repurchased	(12,072)	$	0.86
Unvested restricted common stock as of September 30, 2015	2,013,140	$	0.77

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, 2014	—
Granted	854,691	$	7.91
Exercised	(1,276)	$	7.27
Cancelled or forfeited	(1,166)	$	7.27
Outstanding at September 30, 2015	852,249	$	7.91	9.8	$	1,338
Exercisable at September 30, 2015	71,238	$	7.35	9.6	$	151
Vested and expected to vest at September 30, 2015	852,249	$	7.91	9.8	$	1,338

Employees and directors stock options

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

Assumptions



	Three Months Ended		Nine Months Ended
	September 30, 2015		September 30, 2015
Risk-free interest rate	1.7	%	1.6	%
Expected dividend yield	—		—
Expected term (in years)	6.0		6.0
Expected volatility	78.4	%	78.6	%

Non-employees stock options

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

Assumptions



	As of
	September 30, 2015
Risk-free interest rate	2.2	%
Expected dividend yield	—
Expected term (in years)	10.0
Expected volatility	82.6	%

Summary of significant accounting policies (Details) $ / shares in Units, $ in Millions	Nov. 16, 2015 USD ($) $ / shares shares	Nov. 10, 2015 shares	Oct. 29, 2015	Sep. 30, 2015 shares	Dec. 31, 2014 shares
Commmon stock sales				1,369,715	814,834
Reverse split ratio			4.25
Shares issued upon conversion of redeemable convertible preferred stock		17,647,054
IPO
Commmon stock sales	5,750,000
Price per share \| $ / shares	$ 14.00
Estimated net proceeds \| $	$ 72.7

Summary of significant accounting policies (Details 2) - USD ($)	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Schedule of Investments [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 66,533,000	$ 7,035,000	$ 6,330,000	$ 135,000
Total Money Market Funds And Marketable Securities Fair Value	159,431,000
Marketable securities	94,060,000
Total Money Market Funds And Marketable Securities Amortized Cost	159,393,000
Gross Unrealized Gain	38,000
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Due in one year or less	41,138,000
Due after one year through two years	52,922,000
Total marketable securities	94,060,000
Marketable Securities [Member]
Schedule of Investments [Line Items]
Marketable Securities, Amortized Cost Basis	94,022,000
Marketable securities	94,060,000
Gross Unrealized Gain	38,000
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Total marketable securities	94,060,000
Money market funds
Schedule of Investments [Line Items]
Cash and Cash Equivalents, at Carrying Value	65,371,000
Cash and Cash Equivalents, Fair Value Disclosure	65,371,000
Deposits and other non-current assets
Restricted Cash and Investments [Abstract]
Restricted cash	314,000	$ 314,000
U.S. Treasury notes \| Marketable Securities [Member]
Schedule of Investments [Line Items]
Marketable Securities, Amortized Cost Basis	50,152,000
Marketable securities	50,175,000
Gross Unrealized Gain	23,000
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Total marketable securities	50,175,000
U. S. Government agency bonds \| Marketable Securities [Member]
Schedule of Investments [Line Items]
Marketable Securities, Amortized Cost Basis	43,870,000
Marketable securities	43,885,000
Gross Unrealized Gain	15,000
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Total marketable securities	$ 43,885,000

Summary of significant accounting policies (Details 3) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2015	Dec. 31, 2014
Other comprehensive loss	$ 58,000	$ 38,000
Deposits and other non-current assets
Deferred issuance costs	$ 1,302,000	$ 1,302,000	$ 0

Summary of significant accounting policies (Details 4) - shares	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share (in common stock equivalent shares)	20,512,443	8,387,608
Series A preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share (in common stock equivalent shares)	10,588,235	5,882,352
Series B preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share (in common stock equivalent shares)	7,058,819
Restricted stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share (in common stock equivalent shares)	2,013,140	2,505,256
Outstanding stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share (in common stock equivalent shares)	852,249

Summary of significant accounting policies (Details 5) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share, Pro Forma [Abstract]
Net loss attributable to common stockholders	$ (6,914,000)	$ (4,411,000)	$ (29,473,000)	$ (12,486,000)
Add: Changes in fair value of preferred stock tranche liability			9,750,000	1,754,000
Add: Accretion of preferred stock to redemption value	2,594,000	406,000	6,045,000	859,000
Add: Accrued dividends on temporary equity	403,000		1,039,000
Pro forma net loss attributable to common stockholders	$ (6,914,000)	$ (4,411,000)	$ (29,473,000)	$ (12,486,000)
Weighted average number of common shares outstanding, basic and diluted (in shares)	1,317,150	684,192	1,161,982	583,294
Shares used to compute pro forma net loss per share attributable to common stockholders, basic (in shares)	1,317,150	684,192	1,161,982	583,294
Proforma Basic And Diluted Net Loss Per Share	$ (5.25)	$ (6.45)	$ (25.36)	$ (21.41)

Fair Value Measurements (Details) $ in Thousands	Dec. 31, 2014 USD ($)
Liabilities, Fair Value Disclosure [Abstract]
Convertible preferred stock tranche liability	$ 6,305
Total liabilities measured at fair value	6,305
Level 3
Liabilities, Fair Value Disclosure [Abstract]
Convertible preferred stock tranche liability	6,305
Total liabilities measured at fair value	$ 6,305

Fair Value Measurements (Details 2) - Series A Preferred Stock Tranche Liability - Probability Weighted Present Value Model $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2014 USD ($) $ / shares
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Probability of closing tranche	67.00%
Estimated future value at closing \| $ / shares	$ 1.51
Investment required \| $	$ 20
Discount Rate	(16.20%)

Fair Value Measurements (Details 3) $ in Thousands	9 Months Ended
Fair Value Measurements (Details 3) $ in Thousands	Sep. 30, 2015 USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning	$ 6,305
Changes in fair value	9,750
Reclassification to Series A Preferred Stock (unaudited)	(16,055)
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	159,431
Level 1
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	115,523
Level 2
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	43,870
U.S. Treasury notes
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	50,175
U.S. Treasury notes \| Level 1
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	50,152
U. S. Government agency bonds
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	43,885
U. S. Government agency bonds \| Level 2
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	43,870
Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	65,371
Money market funds \| Level 1
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 65,371

Prepaid expenses and other current assets (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Prepaid expense and other current assets
Prepaid expenses	$ 679	$ 900
Other current assets	263	423
Total	$ 942	$ 1,323

Property and equipment, net (Details) - USD ($)	3 Months Ended		9 Months Ended
Property and equipment, net (Details) - USD ($)	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 3,897,000		$ 3,897,000		$ 2,988,000
Less: accumulated depreciation	(620,000)		(620,000)		(184,000)
Property and equipment, net	3,277,000		3,277,000		2,804,000
Depreciation expense	63,000	$ 160,000	436,000	$ 73,000
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	2,081,000		2,081,000		1,223,000
Furniture and office equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	484,000		484,000		441,000
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 1,332,000		$ 1,332,000		$ 1,324,000

Accrued expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Accrued expenses
Patent costs	$ 250	$ 274
Research and development costs	1,460	125
Professional services	570	81
Employee compensation costs	176	85
Other	486	77
Total	$ 2,942	$ 642

Commitments and contingencies (Details)	1 Months Ended	3 Months Ended		9 Months Ended
Commitments and contingencies (Details)	Mar. 31, 2014 USD ($) item	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Commitments and Contingencies
Number of renewal options \| item	2
Term of each renewal option	5 years
Leasehold incentive received from landlord	$ 1,250,000
Rent expense		$ 403,000	$ 168,000	$ 590,000	$ 140,000
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2015						$ 1,117,000
2016						1,170,000
2017						1,192,000
2018						1,214,000
2019						1,184,000
Total minimum lease payments						$ 5,877,000

Commitments and contingencies (Details 2) - 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
Series B preferred stock
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Fair value of temporary equity issued	25.0
Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment received	65.0
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	$ 645.0
Genzyme \| Series B preferred stock
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Temporary equity issued (in shares) \| shares	10,000,000
Temporary equity issued	$ 30.0
Premium over fair value on temporary equity issued	5.0
Regulatory approval milestone \| Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner	265.0
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
Commerical milestone \| Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner	380.0
Commerical 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
Commerical 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 progam 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 progam specified	$ 20.0

Commitments and contingencies (Details 3) - USD ($)	1 Months Ended	3 Months Ended	9 Months Ended
Commitments and contingencies (Details 3) - USD ($)	Feb. 28, 2015	Sep. 30, 2015	Sep. 30, 2015
Allocable arrangement consideration allocation
Committee Obligations: Alliance Joint Steering Committee ("AJSC")	$ 147,000
Committee Obligations: Development Advisory Committee ("DAC")	227,000
License Option and related deliverables	5,743,000
Total allocable arrangement consideration allocations.	79,256,000
Income Statement Related Disclosures [Abstract]
Collaboration revenue		$ 4,937,000	$ 12,397,000
Huntington's Program
Allocable arrangement consideration allocation
Research and development services	15,662,000
Parkinson's Program
Allocable arrangement consideration allocation
Research and development services	6,648,000
Friedreich's Ataxia Program
Allocable arrangement consideration allocation
Research and development services	16,315,000
Spinal Muscular Atrophy Program
Allocable arrangement consideration allocation
Research and development services	32,050,000
Future Program
Allocable arrangement consideration allocation
Research and development services	2,464,000
Collaborative Arrangement
Allocable arrangement consideration
Non-refundable upfront payment received	65,000,000
Additional consideration, premium over fair value on temporary equity issued	5,000,000
Genzyme \| Collaborative Arrangement
Allocable arrangement consideration
Non-refundable upfront payment received	65,000,000
In-kind services and other funding to be provided by collaborative partner	9,300,000
Income Statement Related Disclosures [Abstract]
Collaboration revenue			12,397,000
Deferred Revenue		$ 59,794,000	$ 59,794,000
Genzyme \| Collaborative Arrangement \| Series B preferred stock
Allocable arrangement consideration
Additional consideration, premium over fair value on temporary equity issued	$ 5,000,000

Commitments and contingencies (Details 4) - USD ($)	1 Months Ended		9 Months Ended	12 Months Ended
Commitments and contingencies (Details 4) - USD ($)	Nov. 30, 2014	Jan. 31, 2014	Sep. 30, 2015	Dec. 31, 2014
University Of Massachusetts (UMass) Collaboration Agreement \| Common Stock [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Issuance of common stock (in shares)		23,529
Issuance of common stock		$ 12,000
Umass and MassBiologics Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount funded to collaborative partner for research and devlopment services to be performed	$ 2,861,000
Term of funding to collaborative partner for research and devlopment services to be performed	16 months
Cumulative funding provided to collaborative partner for research and devlopment services			$ 1,058,000	$ 376,000
Cumulative funding in excess of costs incurred for research and devlopment services performed by collaborative partner			$ 701,000	$ 376,000

Commitments and contingencies (Details 5)	9 Months Ended		12 Months Ended
Commitments and contingencies (Details 5)	Sep. 30, 2015 USD ($) item	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($) item
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	$ 12,000,000
Aggregate upfront fees to be paid to collaborative partner	$ 1,500,000
Number of milestones achieved \| item	0		0
Patent costs incurred	$ 131,000	$ 740,000	$ 839,000
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	$ 75,000	$ 800,000	$ 830,000

Redeemable Convertible Preferred Stock (Details) - USD ($)	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Temporary Equity [Line Items]
Exchange of outstanding convertible notes and accrued interest into Series A redeemable convertible preferred stock and tranche rights		$ 2,929,000
Series A preferred stock
Temporary Equity [Line Items]
Redeemable convertible referred stock, authorized shares	45,000,000		45,000,000
Redeemable convertible preferred stock, issued shares	45,000,000		25,000,000
Redeemable convertible preferred stock, outstanding shares	45,000,000		25,000,000
Price per share	$ 1.00
Proceeds from issuance of stock, net of issuance costs and exchange of outstanding convertible notes	$ 42,039,000
Proceeds from issuance of stock, net of issuance costs	32,000
Exchange of outstanding convertible notes and accrued interest into Series A redeemable convertible preferred stock and tranche rights	2,929,000
Redeemable convertible preferred stock, aggregate liquidation preference (in dollars)	$ 48,621		$ 26,086
Series B preferred stock
Temporary Equity [Line Items]
Redeemable convertible referred stock, authorized shares	30,000,001		0
Redeemable convertible preferred stock, issued shares	30,000,001		0
Redeemable convertible preferred stock, outstanding shares	30,000,001		0
Price per share	$ 3.00
Proceeds from issuance of stock, net of issuance costs	$ 89,780,000
Issuance costs	220,000
Discount recorded against proceeds received for temporary equity related to payment in excess over fair value	5,000,000
Redeemable convertible preferred stock, aggregate liquidation preference (in dollars)	$ 93,827		$ 0

Redeemable Convertible Preferred Stock (Details 2) - USD ($)	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Fair value of Tranche Rights
Fair value of Tranche Right Liability				$ (6,305,000)
Total Fair value of Tranche Right Asset (Liability)				(6,305,000)
Change in fair value of preferred stock tranche liability		$ 9,750,000	$ 1,754,000
Accrued dividends on temporary equity	$ 403,000	1,039,000
Tranche Right I
Fair value of Tranche Rights
Proceeds from issuance of stock, net of issuance costs				18,491,000
Other financing expense \| Tranche Right I
Fair value of Tranche Rights
Change in fair value of preferred stock tranche liability				261,000
Other financing expense \| Tranche Right II
Fair value of Tranche Rights
Change in fair value of preferred stock tranche liability		$ 9,750,000		2,210,000
Series A preferred stock
Temporary Equity [Line Items]
Price per share	$ 1.00	$ 1.00
Fair value of Tranche Rights
Total Fair value of Tranche Right Asset (Liability)	$ (2,600,000)	$ (2,600,000)
Proceeds from issuance of stock, net of issuance costs		32,000
Redeemable convertible preferred stock, initial carrying amount	$ 59,863,000	$ 59,863,000		$ 21,979,000
Series A preferred stock \| 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	$ 1.00
Fair value of Tranche Rights
Fair value of Tranche Right Asset	$ 1,495,000	$ 1,495,000
Issuance of stock (in shares)				18,500,000
Series A preferred stock \| 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	(4,095,000)	$ (4,095,000)
Redeemable convertible preferred stock, initial carrying amount	$ 36,054,000	36,054,000
Accrued dividends on temporary equity		$ 1,039,000
Series B preferred stock
Temporary Equity [Line Items]
Price per share	$ 3.00	$ 3.00
Fair value of Tranche Rights
Proceeds from issuance of stock, net of issuance costs		$ 89,780,000
Redeemable convertible preferred stock, initial carrying amount	$ 88,995,000	$ 88,995,000

Redeemable Convertible Preferred Stock (Details 3) - USD ($)	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Temporary Equity [Line Items]
Exchange of outstanding convertible notes and accrued interest into Series A redeemable convertible preferred stock and tranche rights		$ 2,929,000
Series B preferred stock
Temporary Equity [Line Items]
Redeemable convertible referred stock, authorized shares	30,000,001		0
Redeemable convertible preferred stock, issued shares	30,000,001		0
Redeemable convertible preferred stock, outstanding shares	30,000,001		0
Price per share	$ 3.00
Proceeds from issuance of stock, net of issuance costs	$ 89,780,000
Issuance costs	220,000
Discount recorded against proceeds received for temporary equity related to payment in excess over fair value	5,000,000
Redeemable convertible preferred stock, aggregate liquidation preference (in dollars)	$ 93,827		$ 0

Redeemable Convertible Preferred Stock (Details 4)	9 Months Ended
Redeemable Convertible Preferred Stock (Details 4)	Sep. 30, 2015 USD ($) director Vote $ / shares
Temporary Equity [Line Items]
Minimum gross proceeds from IPO that triggers conversion of temporary equity \| $	$ 50.0
Minimum vote or written consent from outstanding temporary equity shares that triggers conversion of temporary equity (as a percent)	67.00%
Dividend rate (as a percent)	8.00%
Dividends declared \| $	$ 0
Aggregate cumulative preferred dividends \| $	$ 7,448,000
Number of votes for each equivalent common share of temporary equity \| Vote	1
Number of directors temporary equity holders may elect \| director	2
Series A preferred stock
Temporary Equity [Line Items]
Temporary equity conversion price (in dollars per share)	$ 4.25
Temporary equity liquidation preference (in dollars per share)	1.00
Minimum aggregate liquidation preference payments to temporary equity holders that trigger specified alternate preference payout (in dollars per share)	2.50
Series B preferred stock
Temporary Equity [Line Items]
Temporary equity conversion price (in dollars per share)	12.75
Temporary equity redemption price (in dollars per share)	$ 3.00

Common Stock (Details)	9 Months Ended
Common Stock (Details)	Sep. 30, 2015 Vote $ / shares shares	Dec. 31, 2014 $ / shares shares
Class of Stock [Line Items]
Common Stock, Shares Authorized	95,000,000	65,000,000
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)	20,604,977	13,458,104
Preferred Stock outstanding \| Series A preferred stock
Class of Stock [Line Items]
Shares available for future issuance (in shares)	10,588,235	5,882,352
Preferred Stock outstanding \| Series B preferred stock
Class of Stock [Line Items]
Shares available for future issuance (in shares)	7,058,819
Preferred Stock future issuance \| Series A preferred stock
Class of Stock [Line Items]
Shares available for future issuance (in shares)		4,705,883
Vesting of restricted stock awards \| Founder Awards
Class of Stock [Line Items]
Shares available for future issuance (in shares)	889,715	1,068,383
Vesting of restricted stock awards \| 2014 Stock Option and Stock Plan
Class of Stock [Line Items]
Shares available for future issuance (in shares)	1,123,425	1,510,434
Exercise of stock options
Class of Stock [Line Items]
Shares available for future issuance (in shares)	852,249
Issuance under share based compensation plans \| 2014 Stock Option and Grant Plan
Class of Stock [Line Items]
Shares available for future issuance (in shares)	92,534	291,052

Stock based compensation (Details) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
Stock based compensation (Details) - USD ($)	Jan. 31, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Aug. 31, 2015	Apr. 30, 2015	Aug. 31, 2014	Apr. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)		20,604,977		20,604,977		13,458,104
Stock-based compensation		$ 857,000	$ 168,000	$ 2,061,000	$ 275,000
2014 Stock Option and Grant Plan (2014 Plan)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for stock-based compensation plan (in shares)	823,529						2,669,411	2,047,058	2,000,000	1,411,764
Restricted stock granted, original issued price (in dollars per share)				$ 0.04
Shares available for future issuance (in shares)		92,534		92,534
Employees restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation		$ 134,000		$ 325,000
Employees restricted stock \| 2014 Stock Option and Grant Plan - Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock granted (in shares)				1,597,988
Non-employee consultants restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation		457,000		$ 1,119,000
Non-employee consultants restricted stock \| 2014 Stock Option and Grant Plan - Non-employee Consultants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock granted (in shares)				110,960
Founders restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock granted (in shares)	1,188,233
Restricted stock granted, original issued price (in dollars per share)	$ 0.0425
Stock-based compensation				$ 0		$ 0
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]
Stock options granted (in shares)				854,691		0
Employees and directors stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation		215,000		$ 352,000
Employees and directors stock options \| 2014 Stock Option and Grant Plan (2014 Plan)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				4 years
Stock options granted (in shares)				794,309
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		$ 49,000		$ 263,000
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)				60,382
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

Stock based compensation (Details 2) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation	$ 857	$ 168	$ 2,061	$ 275
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation	627	134	1,486	212
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation	$ 230	$ 34	$ 575	$ 63

Stock based compensation (Details 3) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
Stock based compensation (Details 3) - USD ($)	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Restricted stock disclosures
Stock-based compensation	$ 857,000	$ 168,000	$ 2,061,000	$ 275,000
Restricted stock
Shares
Balance, beginning (in shares)			2,578,817
Vested (in shares)			(553,605)
Repurchased (in shares)			(12,072)
Balance, ending (in shares)	2,013,140		2,013,140		2,578,817
Weighted Average Grant Date Fair Value Per Share
Balance, beginning (in dollars per share)			$ 0.77
Vested (in dolalrs per share)			0.73
Repurchased (in dollars per share)			0.86
Balance, ending (in dollars per share)	$ 0.77		$ 0.77		$ 0.77
Restricted stock disclosures
Unrecognized stock-based compensation expense	$ 5,064,000		$ 5,064,000
Remaining weighted-average remaining vesting period			2 years 4 months 21 days
Aggregate fair value of restricted stock vested			$ 2,658,000		$ 253,000
Employees restricted stock
Restricted stock disclosures
Stock-based compensation	134,000		325,000
Non-employee consultants restricted stock
Restricted stock disclosures
Stock-based compensation	$ 457,000		$ 1,119,000

Stock based compensation (Details 4) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
Stock based compensation (Details 4) - USD ($)	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Options disclosures
Stock-based compensation	$ 857,000	$ 168,000	$ 2,061,000	$ 275,000
Stock options, all inclusive
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Granted (in shares)			854,691		0
Exercised (in shares)			(1,276)
Cancelled or forfeited (in shares)			(1,166)
Outstanding, ending balance (in shares)	852,249		852,249
Exerciseable (in shares)	71,238		71,238
Vested and expected to vest (in shares)	852,249		852,249
Weighted Average Exercise Price
Granted (in dollars per share)			$ 7.91
Exercised (in dollars per share)			7.27
Cancelled or forfeited (in dollars per share)			7.27
Outstanding (in dollars per share)	$ 7.91		7.91
Exerciseable (in dollars per share)	7.35		7.35
Vested and expected to vest (in dollars per share)	$ 7.91		$ 7.91
Remaining Contractual Life
Outstanding			9 years 9 months 18 days
Exerciseable			9 years 7 months 6 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Remaining Contractual Term			9 years 9 months 18 days
Aggregate Instrinsic Value
Oustanding	$ 1,338,000		$ 1,338,000
Exerciseable	151,000		151,000
Vested and expected to vest	$ 1,338,000		$ 1,338,000
Employees and directors stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 5.66		$ 5.39
Stock-based compensation	$ 215,000		$ 352,000
Non-employees stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 6.90		$ 6.23
Stock-based compensation	$ 49,000		$ 263,000

Stock based compensation (Details 5) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
Stock based compensation (Details 5) - USD ($)	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Options disclosures
Stock-based compensation	$ 857,000	$ 168,000	$ 2,061,000	$ 275,000
Stock options, all inclusive
Options disclosures
Stock options granted (in shares)			854,691		0
Unrecognized stock-based compensation expense	$ 4,084,000		$ 4,084,000
Remaining weighted-average remaining vesting period			3 years 6 months 18 days
Employees and directors stock options
Fair value assumptions
Risk-free interest rate	1.70%		1.60%
Expected term	6 years		6 years
Expected volatility	78.40%		78.60%
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 5.66		$ 5.39
Stock-based compensation	$ 215,000		$ 352,000
Non-employees stock options
Fair value assumptions
Risk-free interest rate	2.20%
Expected term	10 years
Expected volatility	82.60%
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 6.90		$ 6.23
Stock-based compensation	$ 49,000		$ 263,000

Related party transactions (Details) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended
Related party transactions (Details) - USD ($)	Jan. 31, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Investor \| Consulting and management services
Related Party Transaction [Line Items]
Common stock issued as partial compensation for consulting and management services (in shares)	470,589
Fair value of common stock issued as partial compensation for consulting and management services	$ 238,000
Total amount of services received		$ 6,000	$ 117,000	$ 115,000	$ 1,063,000
Accounts payable to related party		$ 6,000		6,000
Genzyme \| Collaboration arrangement
Related Party Transaction [Line Items]
Revenue recognized				12,397,000
Expense recognized				$ 2,200,000

Subsequent events (Details) $ / shares in Units, $ in Millions				1 Months Ended
	Nov. 16, 2015 USD ($) $ / shares shares	Nov. 10, 2015 shares	Oct. 29, 2015	Oct. 31, 2015 shares	Sep. 30, 2015 shares	Aug. 31, 2015 shares	Apr. 30, 2015 shares	Dec. 31, 2014 shares	Aug. 31, 2014 shares	Apr. 30, 2014 shares	Jan. 31, 2014 shares
Subsequent Event [Line Items]
Reverse split ratio			4.25
Common Stock, Shares, Issued					1,369,715			814,834
Shares issued upon conversion of redeemable convertible preferred stock		17,647,054
2014 Stock Option and Grant Plan (2014 Plan)
Subsequent Event [Line Items]
Shares reserved for issuance (in shares)						2,669,411	2,047,058		2,000,000	1,411,764	823,529
Subsequent Event \| 2014 Stock Option and Grant Plan (2014 Plan)
Subsequent Event [Line Items]
Shares reserved for issuance (in shares)				2,998,823
Subsequent Event \| 2015 Stock Option and Incentive Plan
Subsequent Event [Line Items]
Shares reserved for issuance (in shares)				1,311,812
Increase in percentage of shares issued under option plan				0.04
Common stock shares authorized for issuance				262,362
IPO
Subsequent Event [Line Items]
Common Stock, Shares, Issued	5,750,000
Price per share \| $ / shares	$ 14.00
Estimated net proceeds \| $	$ 72.7