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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2016	May 06, 2016
Document and Entity Information
Entity Registrant Name	Voyager Therapeutics, Inc.
Entity Central Index Key	0001640266
Document Type	10-Q
Document Period End Date	Mar. 31, 2016
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		26,749,740
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	Q1

Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Assets, Current
Cash and cash equivalents	$ 33,895	$ 31,309
Marketable securities	170,097	163,028
Prepaid Expense and Other Assets, Current	1,706	1,557
Total Current Assets	205,698	195,894
Property, Plant and Equipment, Net	3,558	3,234
Deposits and other non-current assets	735	321
Marketable securities	9,989	30,008
Total Assets	219,980	229,457
Liabilities, Current
Accounts Payable	1,697	612
Accrued expenses	2,661	3,430
Deferred rent, current portion		300
Deferred revenue, current portion	18,719	19,589
Total current liabilities	23,077	23,931
Deferred rent, net of current portion	1,352	1,015
Deferred revenue, net of current portion	31,914	35,393
Other non-current liabilities	40	44
Total Liabilities	56,383	60,383
Stockholders' Equity Attributable to Parent
Common stock, $0.001 par value: 120,000,000 shares authorized at March 31, 2016 and December 31, 2015; 25,123,397 and 24,930,979 shares issued and outstanding at March 31, 2016 and December 31, 2015, respectively	25	25
Additional Paid in Capital	220,552	219,122
Accumulated other comprehensive gain	30	(251)
Accumulated deficit	(57,010)	(49,822)
Total stockholders’ deficit	163,597	169,074
Total liabilities, redeemable convertible preferred stock and stockholders’ deficit	$ 219,980	$ 229,457

Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Preferred Stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred Stock, Shares Authorized	5,000,000	5,000,000
Preferred Stock, Shares Issued	0	0
Preferred Stock, Shares Outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common Stock, Shares Authorized	120,000,000	120,000,000
Common Stock, Shares, Issued	25,123,397	24,930,979
Common Stock, Shares, Outstanding	25,123,397	24,930,979

Statement of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Income Statement [Abstract]
Collaboration revenue	$ 4,830	$ 2,576
Operating Expenses [Abstract]
Research and development	8,732	5,523
General and administrative	3,565	1,881
Total operating expenses	12,297	7,404
Operating loss	(7,467)	(4,828)
Other expense, net
Interest income (expense), net	465
Other financing expense	(186)	(9,749)
Total other expense, net	279	(9,749)
Net loss	(7,188)	(14,577)
Other comprehensive loss
Net unrealized loss on available for-sale securities	281
Total other comprehensive loss	281
Comprehensive loss	(6,907)	(14,577)
Reconciliation of net loss to net loss attributalbe to common stockholder:
Accretion of redeemable convertible preferred stock to redemption value		(999)
Accrued dividends on series A preferred stock		(237)
Net loss attributable to common stockholders	$ (7,188)	$ (15,813)
Net loss per share attributable to common stock, basic and diluted	$ (0.29)	$ (15.79)
Weighted-average common shares outstanding, basic and diluted	25,076,769	1,001,370

Statement of Cash Flows - USD ($) $ in Thousands	3 Months Ended
Statement of Cash Flows - USD ($) $ in Thousands	Mar. 31, 2016	Mar. 31, 2015
Cash flow from operating activities
Net loss	$ (7,188)	$ (14,577)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	1,422	302
Depreciation	136	131
Amortization of premiums and discounts on marketable securities	231
Change in fair value of preferred stock tranche liability		9,750
In kind Research And Development Expenses		1,006
Deferred rent	37	(67)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(149)	494
Other non-current assets	57	(51)
Deferred revenue	(4,349)	67,425
Accounts payable	1,085	25
Accrued expenses	(769)	182
Other non-current liabiliites		(170)
Lease incentive benefit		138
Net cash provided by (used in) operating activities	(9,487)	64,588
Cash flow from investing activities
Purchases of property and equipment	(460)	(150)
Change in restricted cash	(471)
Proceeds from marketable securities	13,000
Net cash used in investing activities	12,069	(150)
Cash flow from financing activities
Proceeds from issuance of Series A redeemable convertible preferred stock and tranche rights, net of issuance costs		44,941
Proceeds from issuance of common stock and restricted stock	4
Net cash provided by financing activities	4	44,941
Net increase in cash and cash equivalents	2,586	109,379
Cash and cash equivalents, beginning of period	31,309	7,035
Cash and cash equivalents, end of period	$ 33,895	116,414
Supplemental Disclosure of Cash and non-cash Activities
Accretion of redeemable convertible preferred stock to redemption value		$ 999

Nature of business	3 Months Ended
Nature of business	Mar. 31, 2016
Nature of business
Nature of business	VOYAGER THERAPEUTICS INC. NOTES TO UNAUDITED CONDENSED CONSOLIDATED 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 seven CNS indications, including advanced Parkinson’s disease, a form of monogenic amyotrophic lateral sclerosis, Friedreich’s ataxia, Huntington’s disease, spinal muscular atrophy, frontotemporal dementia / Alzheirmer’s disease, and severe, chronic pain. 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 fourth quarter 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, ability to transition from pilot‑scale manufacturing to large‑scale production of products, and reliance on external sources for financing.

Summary of significant accounting policies	3 Months Ended
Summary of significant accounting policies	Mar. 31, 2016
Summary of significant accounting policies
Summary of significant accounting policies	2. Summary of significant accounting policies and basis of presentation Basis of Presentation The accompanying unaudited condensed consolidated 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 Annual Report on Form 10-K for the year ended December 31, 2015 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”). The accompanying condensed consolidated financial statements include those of the Company and its subsidiary, Voyager Securities Corporation, after elimination of all intercompany accounts and transactions. In November 2015, the Company completed the sale of 5,750,000 shares of its common stock in its initial public offering (the “IPO”), at a price to the public of $14.00 per share, resulting in net proceeds to the Company of $72.9 million after deducting underwriting discounts, commissions, and offering expenses paid 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. These condensed consolidated financial statements should be read in conjunction with the Company's audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2015 as filed with the SEC. 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 prior to the closing of the IPO, 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. 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, Presentations of Financial Statements, Going Concern, 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. The adoption of this accounting standard may affect our financial statement disclosures in future periods. The Company is evaluating the impact that this ASU may have on its financial statements, if any. In February 2016, the FASB issued ASU 2016-02, Leases, which requires a lessee to recognize assets and liabilities on the balance sheet for operating leases and changes many key definitions, including the definition of a lease. The new standard includes a short-term lease exception for leases with a term of 12 months or less, as part of which a lessee can make an accounting policy election not to recognize lease assets and lease liabilities. Lessees will continue to differentiate between finance leases (previously referred to as capital leases) and operating leases using classification criteria that are substantially similar to the previous guidance. The new standard will be effective beginning January 1, 2019, and early adoption is permitted for public entities. The Company is currently evaluating the potential impact this ASU may have on its financial position.

Fair Value Measurements

3 Months Ended

Mar. 31, 2016

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 March 31, 2016 and December 31, 2015 are as follows:


			Quoted Prices		Significant
			in Active		Other		Significant
			Markets for		Observable		Unobservable
			Identical Assets		Inputs		Inputs
Assets	Total		(Level 1)		(Level 2)		(Level 3)
March 31, 2016	(in thousands)
Money market funds included in cash and cash equivalents	$	33,035	$	33,035	$	—	$	—
Marketable securities:
U.S. Treasury notes		153,060		153,060		—		—
U.S. Government agency securities		27,026		—		27,026		—
Total	$	213,121	$	186,095	$	27,026	$	—
December 31, 2015
Money market funds included in cash and cash equivalents	$	29,601	$	29,601	$	—	$	—
Marketable securities:
U.S. Treasury notes		157,981		157,981		—		—
U.S. Government agency securities		35,055		—		35,055		—
Total	$	222,637	$	187,582	$	35,055	$	—

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.

- 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

Cash, Cash Equivalents, and Available for Sale Marketable Investments

3 Months Ended

Mar. 31, 2016

Cash and Cash Equivalents [Abstract]

Cash, Cash Equivalents, and Available for Sale Marketable Investments

4. Cash, Cash Equivalents, and Available for Sale Marketable Investments

Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Investments consist of securities with original maturities greater than 90 days when purchased. The Company classifies these investments as available-for-sale and records them at fair value in the accompanying condensed consolidated balance sheets. Unrealized gains or losses are included in accumulated other comprehensive income (loss). Premiums or discounts from par value are amortized to investment income over the life of the underlying investment.

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 12 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/(loss) 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. No other temporary losses are recognized.

Cash, cash equivalents and investments, available for sale included the following at March 31, 2016 and December 31, 2015:


	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
	(in thousands)
As of March 31, 2016
Money market funds included in cash and cash equivalents	$	33,035	$	—	$	—	$	33,035
Total money market funds included in cash and cash equivalents	$	33,035	$	—	$	—	$	33,035
Marketable securities:
U.S. Treasury notes		153,027		33		—		153,060
U.S. Government agency bonds		27,029		—		3		27,026
Total marketable securities	$	180,056	$	33	$	3	$	180,086
Total money market funds and marketable securities	$	213,091	$	33	$	3	$	213,121
As of December 31, 2015
Money market funds included in cash and cash equivalents	$	29,601	$	—	$	—	$	29,601
Total money market funds included in cash and cash equivalents	$	29,601	$	—	$	—	$	29,601
Marketable securities:
U.S. Treasury notes		158,166		—		185		157,981
U.S. Government agency bonds		35,121		—		66		35,055
Total marketable securities	$	193,287	$	—	$	251	$	193,036
Total money market funds and marketable securities	$	222,888	$	—	$	251	$	222,637

The estimated fair value of the Company’s marketable securities balance at March 31, 2016, by contractual maturity, is as follows


Due in one year or less	$	170,097
Due after one year through two years		9,989
Total marketable securities	$	180,086

- Definition

The entire disclosure for cash and cash equivalent footnotes, which may include the types of deposits and money market instruments, applicable carrying amounts, restricted amounts and compensating balance arrangements. Cash and equivalents include: (1) currency on hand (2) demand deposits with banks or financial institutions (3) other kinds of accounts that have the general characteristics of demand deposits (4) short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Generally, only investments maturing within three months from the date of acquisition qualify.

+ References + Details

Accrued expenses

3 Months Ended

Mar. 31, 2016

Accrued expenses

Accrued expenses

. Accrued Expenses

Accrued expenses as of March 31, 2016 and December 31, 2015 consist of the following:


	As of March 31,		As of December 31,
	2016		2015
	(in thousands)
Research and development costs	$	1,393	$	1,329
Employee compensation costs		639		1,338
Professional services		374		350
Other		215		178
Patent costs		40		235
Total	$	2,661	$	3,430

Commitments and contingencies

3 Months Ended

Mar. 31, 2016

Commitments and Contingencies

Commitments and contingencies

6. 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 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.

In January 2016, the Company executed an amendment to extend the lease to December 2024 to coincide with an agreement to lease an additional facility of approximately 26,000 square feet in Cambridge, Massachusetts. Estimated payments related to the lease amendment and additional facility are approximately $25.0 million. The leases also include two renewal options, each for five year terms and at fair market value upon exercise. The Company will expense rent on a straight‑line basis over the term of the new lease, including any rent‑free periods once the landlord delivery date has commenced. The delivery date is currently set for the third quarter of 2016 and is projected to be ready for occupancy in early 2017.

The Company received a leasehold improvement incentive for the new lease from the landlord totaling $3,530,000. The Company will record 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.

The following table summarizes our significant contractual obligations as of payment due date by period at March 31, 2016 (in thousands):


	Total Minimum
	Lease Payments
	(in thousands)
2016		886
2017		3,211
2018		3,290
2019		3,382
2020		3,762
2021+		16,141
	$	30,672

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 Parkinson’s Program, the Huntington’s Program, or generally with the manufacture of Split Territory Program products.

The Company is solely responsible for all costs incurred in connection with the development of the Split Territory Programs and the Spinal Muscular Atrophy Program products prior to the exercise of an option by 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 licensed product from the Spinal Muscular Atrophy Program (the “Spinal Muscular Atrophy Licensed 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 Licensed 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 applicable 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 each Split Territory Licensed Product and the Spinal Muscular Atrophy 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 three month period ended March 31, 2016, the Company recognized $4,830,000 of revenue associated with its collaboration with Genzyme related to research and development services performed during the period. As of March 31, 2016, there is $50,633,000 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 three months ended March 31, 2016. The Company estimates that the majority of research and development expense during the period relate to programs for which Genzyme has an option right.

Litigation

The Company is not a party to any litigation and does not have contingency reserves established for any litigation liabilities as of March 31, 2016 or December 31, 2015.

Redeemable Convertible Preferred Stock	3 Months Ended
Redeemable Convertible Preferred Stock	Mar. 31, 2016
Redeemable Convertible Preferred Stock Abstract
Redeemable Convertible Preferred Stock [Texl Block]	7. Redeemable Convertible Preferred Stock In November 2015, upon the closing of the Company’s IPO, all issued and outstanding redeemable convertible preferred stock was automatically converted into 17,647,054 shares of common stock. The Company has newly authorized preferred stock amounting to 5,000,000 shares as of March 31, 2016 and December 31, 2015. The newly authorized preferred stock was classified under stockholders’ equity (deficit) at March 31, 2016 and December 31, 2015.

Stock-based compensation

3 Months Ended

Mar. 31, 2016

Stock based compensation

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

8. Stock‑Based Compensation

2015 Stock Option Plan

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

Effective January 1, 2016, an additional 1,069,971 shares were added to the Company’s 2015 Stock Option Plan, for future issuance pursuant to the terms of the 2015 Stock Option and Grant Plan, or the 2014 Plan. As of March 31, 2016, there were 2,098,221 shares of common stock available for future award grants under the 2015 Plan. During the three months ended March 31, 2016, the Company issued a total of 603,310 stock options to employees and directors and 2,500 stock options to non-employees under the 2015 Stock Option Plan.

2014 Stock Option and Grant Plan

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

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. During the three months ended March 31, 2016, management has determined that the achievement of one of the three performance‑based milestones was probable. Accordingly, stock-based compensation expense in the amount of $578,000 was recorded related to this award during the three months ended March 31, 2016. No stock‑based compensation expense was recorded for the remaining two Founders’ awards with performance-based vesting as of March 31, 2016 and December 31, 2015 as the performance-based milestones related to these awards were not probable.

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. A total of 267,492 shares of common stock were subsequently added during the three months ended March 31, 2016.

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
	March 31,
	2016		2015
	(in thousands)
Research and development	$	1,117	$	225
General and administrative		305		77
Total stock-compensation expense	$	1,422	$	302

Restricted Stock

A summary of the status of and changes in unvested restricted stock unit activity under the Company’s equity award plan was as follows:


		Weighted
		Average
		Grant Date
		Fair Value
	Shares	Per Share
Unvested restricted common stock as of December 31, 2015	1,818,261	$	0.76
Issued	—		—
Vested	(191,918)	$	0.73
Repurchased	—		—
Unvested restricted common stock as of March 31, 2016	1,626,343	$	0.76

The expense related to awards granted to employees and non-employees was $132,000 and $727,000, respectively, for the period ended March 31, 2016.

As of March 31, 2016, the Company had unrecognized stock-based compensation expense related to its unvested restricted stock awards of $12,105,000, which is expected to be recognized over the remaining average vesting period of 2.1 years.

Stock Options

The following is a summary of stock option activity for the three months ended March 31, 2016:


		Weighted		Remaining	Aggregate
		Average		Contractual	Intrinsic
		Exercise		Life	Value
	Shares	Price		(in years)	(in thousands)
Outstanding at December 31, 2015	1,022,617	$	8.35
Granted	605,810	$	11.18
Exercised	(500)	$	7.27
Cancelled or forfeited	(13,581)	$	9.53
Outstanding at March 31, 2016	1,614,346	$	9.41	9.5	$	691
Exercisable at March 31, 2016	155,477	$	7.82	9.1	$	170
Vested and expected to vest at March 31, 2016	1,614,346	$	9.41	9.5	$	691

Using the Black‑Scholes option pricing model, the weighted average fair value of options granted to employees and directors during the three months ended March 31, 2016 was $7.22. The expense related to awards granted to employees and directors was $551,000 for the three months ended March 31, 2016. There were no stock options awarded during the three months ended March 31, 2015.

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
	March 31, 2016
Risk-free interest rate	1.5	%
Expected dividend yield	—	%
Expected term (in years)	6.0
Expected volatility	73.0	%

Using the Black‑Scholes option pricing model, the weighted average grant date fair value of options granted to non‑employees during the three months ended March 31, 2016 was $7.10. 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 $12,000 for the three months ended March 31, 2016. There were no stock options awarded during the three months ended March 31, 2015.

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:


	Three Months Ended
	March 31, 2016
Risk-free interest rate	1.5	%
Expected dividend yield	—	%
Expected term (in years)	10.0
Expected volatility	84.1	%

As of March 31, 2016, the Company had unrecognized stock-based compensation expense related to its unvested stock options of $8,556,000 which is expected to be recognized over the remaining weighted average vesting period of 3.35 years.

For the three months ended March 31, 2016 and March 31, 2015, expected volatility was estimated using the historical volatility of the common stock of a group of similar companies that are publicly traded. The Company will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available.

Net loss per share

3 Months Ended

Mar. 31, 2016

Earnings Per Share [Abstract]

Net loss per share

9. Net Loss Per Share

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 March 31,
	2016	2015
Redeemable convertible preferred stock	—	12,941,176
Unvested restricted common stock	1,626,343	2,325,510
Outstanding stock options	1,614,346	—
Total	3,240,689	15,266,686

Related party transactions	3 Months Ended
Related party transactions	Mar. 31, 2016
Related party transactions
Related party transactions	10. 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 $3,000 and $69,000 during the three months ended March 31, 2016 and March 31, 2015, respectively. As of March 31, 2016, the Company included approximately $3,000, in accounts payable related to service fees charged by this investor. During the three month period ended March 31, 2016 the Company recognized $4,830,000 of revenue associated with the Genzyme Collaboration related to research and development services provided during this period. The Company also recognized $481,000 of expense during the three months ended March 31, 2016 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

Summary of significant accounting policies (Policies)	3 Months Ended
Summary of significant accounting policies (Policies)	Mar. 31, 2016
Summary of significant accounting policies
Basis of presentation	Basis of Presentation The accompanying unaudited condensed consolidated 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 Annual Report on Form 10-K for the year ended December 31, 2015 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”). The accompanying condensed consolidated financial statements include those of the Company and its subsidiary, Voyager Securities Corporation, after elimination of all intercompany accounts and transactions.
Use of estimates	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 prior to the closing of the IPO, 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.
Recent accounting pronouncements	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, Presentations of Financial Statements, Going Concern, 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. The adoption of this accounting standard may affect our financial statement disclosures in future periods. The Company is evaluating the impact that this ASU may have on its financial statements, if any. In February 2016, the FASB issued ASU 2016-02, Leases, which requires a lessee to recognize assets and liabilities on the balance sheet for operating leases and changes many key definitions, including the definition of a lease. The new standard includes a short-term lease exception for leases with a term of 12 months or less, as part of which a lessee can make an accounting policy election not to recognize lease assets and lease liabilities. Lessees will continue to differentiate between finance leases (previously referred to as capital leases) and operating leases using classification criteria that are substantially similar to the previous guidance. The new standard will be effective beginning January 1, 2019, and early adoption is permitted for public entities. The Company is currently evaluating the potential impact this ASU may have on its financial position

Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2016

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 March 31, 2016 and December 31, 2015 are as follows:


			Quoted Prices		Significant
			in Active		Other		Significant
			Markets for		Observable		Unobservable
			Identical Assets		Inputs		Inputs
Assets	Total		(Level 1)		(Level 2)		(Level 3)
March 31, 2016	(in thousands)
Money market funds included in cash and cash equivalents	$	33,035	$	33,035	$	—	$	—
Marketable securities:
U.S. Treasury notes		153,060		153,060		—		—
U.S. Government agency securities		27,026		—		27,026		—
Total	$	213,121	$	186,095	$	27,026	$	—
December 31, 2015
Money market funds included in cash and cash equivalents	$	29,601	$	29,601	$	—	$	—
Marketable securities:
U.S. Treasury notes		157,981		157,981		—		—
U.S. Government agency securities		35,055		—		35,055		—
Total	$	222,637	$	187,582	$	35,055	$	—

Cash, Cash Equivalents, and Available for Sale Marketable Investments (Tables)

3 Months Ended

Mar. 31, 2016

Cash and Cash Equivalents [Abstract]

Cash, cash equivalents and investments, available for sale

Cash, cash equivalents and investments, available for sale included the following at March 31, 2016 and December 31, 2015:


	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
	(in thousands)
As of March 31, 2016
Money market funds included in cash and cash equivalents	$	33,035	$	—	$	—	$	33,035
Total money market funds included in cash and cash equivalents	$	33,035	$	—	$	—	$	33,035
Marketable securities:
U.S. Treasury notes		153,027		33		—		153,060
U.S. Government agency bonds		27,029		—		3		27,026
Total marketable securities	$	180,056	$	33	$	3	$	180,086
Total money market funds and marketable securities	$	213,091	$	33	$	3	$	213,121
As of December 31, 2015
Money market funds included in cash and cash equivalents	$	29,601	$	—	$	—	$	29,601
Total money market funds included in cash and cash equivalents	$	29,601	$	—	$	—	$	29,601
Marketable securities:
U.S. Treasury notes		158,166		—		185		157,981
U.S. Government agency bonds		35,121		—		66		35,055
Total marketable securities	$	193,287	$	—	$	251	$	193,036
Total money market funds and marketable securities	$	222,888	$	—	$	251	$	222,637

Estimated fair value of the Company’s marketable securities balance

The estimated fair value of the Company’s marketable securities balance at March 31, 2016, by contractual maturity, is as follows


Due in one year or less	$	170,097
Due after one year through two years		9,989
Total marketable securities	$	180,086

Accrued expenses (Tables)

3 Months Ended

Mar. 31, 2016

Accrued expenses

Schedule of accrued expenses

Accrued expenses as of March 31, 2016 and December 31, 2015 consist of the following:


	As of March 31,		As of December 31,
	2016		2015
	(in thousands)
Research and development costs	$	1,393	$	1,329
Employee compensation costs		639		1,338
Professional services		374		350
Other		215		178
Patent costs		40		235
Total	$	2,661	$	3,430

Commitments and contingencies (Tables)

3 Months Ended

Mar. 31, 2016

Commitments and Contingencies

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

Net loss per share (Tables)

3 Months Ended

Mar. 31, 2016

Earnings Per Share [Abstract]

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


	As of March 31,
	2016	2015
Redeemable convertible preferred stock	—	12,941,176
Unvested restricted common stock	1,626,343	2,325,510
Outstanding stock options	1,614,346	—
Total	3,240,689	15,266,686

Summary of significant accounting policies - Basis of Presentation - (Details) $ / shares in Units, $ in Millions		1 Months Ended
	Nov. 16, 2015 shares	Nov. 30, 2015 USD ($) $ / shares shares	Mar. 31, 2016 shares	Dec. 31, 2015 shares
Commmon stock sales			25,123,397	24,930,979
IPO
Commmon stock sales		5,750,000
Price per share \| $ / shares		$ 14.00
Net proceeds \| $		$ 72.9
Reverse split ratio		4.25
Common Stock
Shares issued upon conversion of redeemable convertible preferred stock	17,647,054	17,647,054

Fair Value Measurements - Assets measured on a recurring basis - (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents		$ 29,601
Total assets measured at fair value	$ 213,121	222,637
Level 1
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents		29,601
Total assets measured at fair value	186,095	187,582
Level 2
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	27,026	35,055
U.S. Treasury notes
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	153,060	157,981
U.S. Treasury notes \| Level 1
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	153,060	157,981
U. S. Government agency bonds
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	27,026	35,055
U. S. Government agency bonds \| Level 2
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	27,026	$ 35,055
Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	33,035
Money market funds \| Level 1
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 33,035

Cash, Cash Equivalents, and Available for Sale Marketable Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Amortized Cost Abstract
Amortized cost	$ 213,091	$ 222,888
Unrealized gains	33
Unrealized losses	3	251
Fair value	213,121	222,637
Money market funds
Amortized Cost Abstract
Amortized cost	33,035	29,601
Fair value	33,035	29,601
Marketable Securities
Amortized Cost Abstract
Amortized cost	180,056	193,287
Unrealized gains	33
Unrealized losses	3	251
Fair value	180,086	193,036
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis, Rolling Maturity [Abstract]
Due in one year or less	170,097
Due after one year through two years	9,989
U.S. Treasury notes
Amortized Cost Abstract
Amortized cost	153,027	158,166
Unrealized gains	33
Unrealized losses		185
Fair value	153,060	157,981
U.S. Government agency bonds
Amortized Cost Abstract
Amortized cost	27,029	35,121
Unrealized losses	3	66
Fair value	$ 27,026	$ 35,055

Accrued expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Accrued expenses
Research and development costs	$ 1,393	$ 1,329
Employee compensation costs	639	1,338
Professional services	374	350
Other	215	178
Patent costs	40	235
Total	$ 2,661	$ 3,430

Commitments and contingencies - Operating Leases - (Details) - USD ($)	Mar. 31, 2016	Mar. 31, 2014
Commitments and Contingencies
Leasehold incentive received from landlord		$ 1,250,000
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2016	$ 886,000
2017	3,211,000
2018	3,290,000
2019	3,382,000
2020	3,762,000
2121 +	16,141,000
Total minimum lease payments	$ 30,672,000

Commitments and contingencies - Significant Agreements - (Details) - Collaborative Arrangement $ in Millions	1 Months Ended
	Feb. 28, 2015 USD ($) shares
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment received	$ 65.0
Premium over fair value on temporary equity issued	5.0
Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment received	65.0
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
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
Redeemable Convertible Preferred Stock Series B
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Fair value of temporary equity issued	$ 25.0
Redeemable Convertible Preferred Stock Series B \| Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Temporary equity issued (in shares) \| shares	10,000,000
Temporary equity issued	$ 30.0
Premium over fair value on temporary equity issued	$ 5.0

Commitments and contingencies - Accounting Analysis - (Details) - USD ($)	1 Months Ended	3 Months Ended
	Feb. 28, 2015	Mar. 31, 2016	Mar. 31, 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		$ 4,830,000	$ 2,576,000
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		4,830,000
Deferred Revenue		$ 50,633,000
Redeemable Convertible Preferred Stock Series B \| Genzyme \| Collaborative Arrangement
Allocable arrangement consideration
Additional consideration, premium over fair value on temporary equity issued	$ 5,000,000

Commitments and contingencies - Other Agreements - (Details)	Mar. 31, 2016 USD ($)
Loss Contingency [Abstract]
Contingency reserves for litigation liabilities	$ 0

Redeemable Convertible Preferred Stock - Series A Preferred Stock- Series A Preferred Stock- (Details) - shares		1 Months Ended
	Nov. 16, 2015	Nov. 30, 2015	Mar. 31, 2016	Dec. 31, 2015
Temporary Equity [Line Items]
Preferred Stock, Shares Authorized			5,000,000	5,000,000
Common Stock
Temporary Equity [Line Items]
Shares issued upon conversion of redeemable convertible preferred stock	17,647,054	17,647,054

Stock based compensation - 2014 Stock Option and Grant Plan - (Details)		1 Months Ended		3 Months Ended		12 Months Ended
	Jan. 01, 2016 shares	Oct. 31, 2015 shares	Jan. 31, 2014 $ / shares shares	Mar. 31, 2016 USD ($) shares	Mar. 31, 2015 USD ($) shares	Dec. 31, 2015 USD ($)	Dec. 31, 2014 shares	Aug. 31, 2015 shares	Apr. 30, 2015 shares	Aug. 31, 2014 shares	Apr. 30, 2014 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation \| $				$ 1,422,000	$ 302,000
Stock options granted (in shares)				605,810	0
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)		2,998,823	823,529					2,669,411	2,047,058	2,000,000	1,411,764
Vesting period		15 days
2014 Stock Option and Grant Plan (2014 Plan) \| Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage		25.00%
2014 Stock Option and Grant Plan (2014 Plan) \| Vesting period, tranche two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage		75.00%
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
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
2015 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)		262,362		267,492
2015 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)		1,311,812		2,098,221
Percentage limit of issued and outstanding shares upto which additional shares may be issued				4.00
Additional shares added to stock option plan	1,069,971
Unvested restricted stock awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock-based compensation expense \| $				$ 12,105,000
Employees restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation \| $				132,000
Non-employee consultants restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation \| $				727,000
Non-employee consultants restricted stock \| 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-employee consultants restricted stock \| 2014 Stock Option and Grant Plan (2014 Plan) \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period		4 years
Performance based milestone awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation \| $				578,000
Founders restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation \| $				0		$ 0
Restricted stock granted (in shares)			1,188,233
Restricted stock granted, original issued price (in dollars per share) \| $ / shares			$ 0.0425
Founders restricted stock \| Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock granted (in shares)			835,292
Founders restricted stock \| Vesting period, tranche two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock granted (in shares)			352,941
Founders restricted stock \| Minimum \| Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			1 year
Founders restricted stock \| Maximum \| Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			4 years
Employees and directors stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation \| $				$ 551,000
Employees and directors stock options \| 2015 Stock Option and Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (in shares)				603,310
Non-employees stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation \| $				$ 12,000
Non-employees stock options \| 2015 Stock Option and Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (in shares)				2,500

Stock based compensation - Founder Awards - (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation	$ 1,422	$ 302
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation	1,117	225
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation	$ 305	$ 77

Stock based compensation - Restricted Stock - (Details) - USD ($)	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Shares
Balance, beginning (in shares)	1,818,261
Vested (in shares)	(191,918)
Balance, ending (in shares)	1,626,343
Weighted Average Grant Date Fair Value Per Share
Balance, beginning (in dollars per share)	$ 0.76
Vested (in dolalrs per share)	0.73
Balance, ending (in dollars per share)	$ 0.76
Restricted stock disclosures
Stock-based compensation	$ 1,422,000	$ 302,000
Unvested restricted stock awards
Restricted stock disclosures
Unrecognized stock-based compensation expense	$ 12,105,000
Remaining weighted-average remaining vesting period	2 years 1 month 6 days
Employees restricted stock
Restricted stock disclosures
Stock-based compensation	$ 132,000
Non-employee consultants restricted stock
Restricted stock disclosures
Stock-based compensation	$ 727,000

Stock based compensation - Stock Options - (Details) - USD ($)	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding, beginning balance (in shares)	1,022,617
Granted (in shares)	605,810	0
Exercised (in shares)	(500)
Cancelled or forfeited (in shares)	(13,581)
Outstanding, ending balance (in shares)	1,614,346
Exerciseable (in shares)	155,477
Vested and expected to vest (in shares)	1,614,346
Weighted Average Exercise Price
Oustanding (in dollars per share)	$ 8.35
Granted (in dollars per share)	11.18
Exercised (in dollars per share)	7.27
Cancelled or forfeited (in dollars per share)	9.53
Oustanding (in dollars per share)	9.41
Exerciseable (in dollars per share)	7.82
Vested and expected to vest (in dollars per share)	$ 9.41
Remaining Contractual Life
Outstanding	9 years 6 months
Exerciseable	9 years 1 month 6 days
Vested and expected to vest	9 years 6 months
Aggregate Instrinsic Value
Oustanding	$ 691,000
Exerciseable	170,000
Vested and expected to vest	691,000
Options disclosures
Stock-based compensation	$ 1,422,000	$ 302,000
Employees and directors stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 7.22
Stock-based compensation	$ 551,000
Non-employees stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 7.10
Stock-based compensation	$ 12,000

Stock based compensation - Black-Scholes options pricing model - (Details) - USD ($)	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Options disclosures
Stock-based compensation	$ 1,422,000	$ 302,000
Stock options granted (in shares)	605,810	0
Employees and directors stock options
Fair value assumptions
Risk-free interest rate	1.50%
Expected term	6 years
Expected volatility	73.00%
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 7.22
Stock-based compensation	$ 551,000
Non-employees stock options
Fair value assumptions
Risk-free interest rate	1.50%
Expected term	10 years
Expected volatility	84.10%
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 7.10
Stock-based compensation	$ 12,000
Unvested stock options
Options disclosures
Unrecognized stock-based compensation expense	$ 8,556,000
Remaining weighted-average remaining vesting period	3 years 4 months 6 days

Net loss per share (Details) - shares	3 Months Ended
Net loss per share (Details) - shares	Mar. 31, 2016	Mar. 31, 2015
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)	3,240,689	15,266,686
Redeemable convertible preferrred 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)		12,941,176
Unvested restricted stock awards
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)	1,626,343	2,325,510
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)	1,614,346

Related party transactions (Details) - USD ($)	1 Months Ended	3 Months Ended
Related party transactions (Details) - USD ($)	Jan. 31, 2014	Mar. 31, 2016	Mar. 31, 2015
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		$ 3,000	$ 69,000
Accounts payable to related party		3,000
Genzyme \| Collaboration arrangement
Related Party Transaction [Line Items]
Revenue recognized		4,830,000
Expense recognized		$ 481,000