Voyager Therapeutics (VYGR) 10-K 17 Mar 16 2015 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2015	Mar. 14, 2016	Jun. 30, 2015
Document and Entity Information
Entity Registrant Name	Voyager Therapeutics, Inc.
Entity Central Index Key	1,640,266
Document Type	10-K
Document Period End Date	Dec. 31, 2015
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	No
Entity Filer Category	Non-accelerated Filer
Entity Public Float			$ 0
Entity Common Stock, Shares Outstanding		26,749,240
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	FY

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Assets, Current
Cash and cash equivalents	$ 31,309	$ 7,035
Marketable securities	163,028
Prepaid Expense and Other Assets, Current	1,557	1,323
Total Current Assets	195,894	8,358
Property, Plant and Equipment, Net	3,234	2,804
Deposits and other non-current assets	321	335
Marketable securities	30,008
Total Assets	229,457	11,497
Liabilities, Current
Accounts Payable	612	1,554
Accrued expenses	3,430	642
Deferred rent, current portion	300	278
Deferred revenue, current portion	19,589
Total current liabilities	23,931	2,474
Deferred rent, net of current portion	1,015	1,314
Deferred revenue, net of current portion	35,393
Other non-current liabilities	44	255
Preferred stock tranche liability		6,305
Total Liabilities	60,383	10,348
Stockholders' Equity Attributable to Parent
Common stock, $0.001 par value: 105,000,000 and 65,000,000 shares authorized at December 31, 2015 and December 31, 2014 respectively; 24,930,979 and 814,834 shares issued and outstanding at December 31, 2015 and December 31, 2014, respectively	25	1
Additional Paid in Capital	219,122
Accumulated other comprehensive gain	(251)
Accumulated deficit	(49,822)	(20,831)
Total stockholders’ deficit	169,074	(20,830)
Total liabilities, redeemable convertible preferred stock and stockholders’ deficit	229,457	11,497
Redeemable Convertible Preferred Stock Series A
Liabilities, Current
Preferred stock tranche liability	$ 2,600
Redeemable convertible preferred stock		$ 21,979

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Redeemable convertible preferred stock, outstanding shares		25,000,000
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common Stock, Shares Authorized	120,000,000	65,000,000
Common Stock, Shares, Issued	24,930,979	814,834
Common Stock, Shares, Outstanding	24,930,979	814,834
Redeemable Preferred Stock
Preferred Stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred Stock, Shares Authorized	5,000,000	0
Preferred Stock, Shares Issued	0	0
Preferred Stock, Shares Outstanding	0	0
Redeemable Convertible Preferred Stock
Redeemable convertible preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Redeemable convertible referred stock, authorized shares	0	45,000,000
Redeemable convertible preferred stock, issued (in shares)	0	25,000,000
Redeemable convertible preferred stock, outstanding shares	0
Redeemable convertible preferred stock, aggregate liquidation preference (in dollars)		$ 26,086
Redeemable Convertible Preferred Stock Series A
Preferred Stock, Shares Authorized	45,000,000	45,000,000
Redeemable Convertible Preferred Stock Series B
Redeemable convertible referred stock, authorized shares	30,000,001
Redeemable convertible preferred stock, issued (in shares)	30,000,001

Statement of Operations and Com

Statement of Operations and Comprehensive Loss - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Collaboration revenue		$ 17,334
Operating Expenses [Abstract]
Research and development	$ 2,316	27,679	$ 8,898
General and administrative	1,450	9,909	5,469
Total operating expenses	3,766	37,588	14,367
Operating loss	(3,766)	(20,254)	(14,367)
Other expense, net
Interest income (expense), net	(67)	332	(1)
Other financing expense		(9,750)	(1,949)
Total other expense, net	(67)	(9,418)	(1,950)
Net loss	(3,833)	(29,672)	(16,317)
Other comprehensive loss
Net unrealized loss on available for-sale securities		(251)
Total other comprehensive loss		(251)
Comprehensive loss	(3,833)	(29,923)	(16,317)
Reconciliation of net loss to net loss attributalbe to common stockholder:
Accretion of redeemable convertible preferred stock to redemption value		(7,373)	(1,366)
Accrued dividends on series A preferred stock		(1,245)
Net loss attributable to common stockholders	$ (3,833)	$ (38,290)	$ (17,683)
Net loss per share attributable to common stock, basic and diluted	$ (1,629.68)	$ (9.14)	$ (27.83)
Weighted-average common shares outstanding, basic and diluted	2,352	4,191,210	635,448

Redeemable Convertible Preferre

Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred Stock Series APreferred Stock	Redeemable Convertible Preferred Stock Series BPreferred StockIssuance Number #1	Redeemable Convertible Preferred Stock Series BPreferred Stock	Redeemable Convertible Preferred Stock Series B	Common StockIPO	Common Stock	Additional Paid-in CapitalIPO	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit	IPO	Total
Stockholders' Equity, Begining Balance at Dec. 31, 2013											$ (3,833)		$ (3,833)
Shares, Outstanding, Beginning Balance at Dec. 31, 2013							2,352
Issuance of common stock for services							$ 1		$ 251				252
Issuance of common stock for services							494,118
Initial issuance of Series A redeemable convertible preferred stock, including exchange of convertible notes payable and net of tranche rights and issuance costs		$ 3,878
Initial issuance of Series A redeemable convertible preferred stock, including exchange of convertible notes payable and net of tranche rights and issuance costs ( in shares)		6,500,000
Issuance of preferred stock, net of discount and issuance costs		$ 18,491
Reclassification of tranche rights upon issuance of Series A redeemable convertible preferred stock		(1,756)
Net loss											(16,317)		(16,317)
Vesting of restricted common stock									9				9
Vesting of restricted common stock (in shares)							318,364
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition									425				425
Stock-based compensation expense									425				425
Accretion of redeemable convertible preferred stock to redemption value (Preferred Stock)		1,366
Accretion of redeemable convertible preferred stock to redemption value									685		681		1,366
Stockholders' Equity, Ending Balance at Dec. 31, 2014		$ 21,979					$ 1				(20,831)		(20,830)
Shares, Outstanding, Ending Balance at Dec. 31, 2014		25,000,000					814,834
Issuance of preferred stock, net of discount and issuance costs (in shares)	25,000,000	18,500,000
Issuance of common stock						$ 6		$ 72,948				$ 72,954
Issuance of common stock (in shares)						5,750,000
Issuance of preferred stock, net of discount and issuance costs		$ 19,999	$ 84,780
Reclassification of tranche rights upon issuance of Series A redeemable convertible preferred stock		16,055
Conversion of redeemable convertible preferred stock to common stock		$ (60,182)		$ (90,005)			$ 18		144,675		5,494		150,187
Conversion of redeemable convertible preferred stock to common stock (common stock shares)							17,647,054
Conversion of redeemable convertible preferred stock to common stock (preferred stock shares)		(45,000,000)		(30,000,001)
Net loss											(29,672)		(29,672)
Vesting of restricted common stock									22				22
Vesting of restricted common stock (in shares)							717,747
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition									4,027				4,027
Stock-based compensation expense									4,027				4,027
Unrealized loss on available for-sale securities										$ (251)			(251)
Accretion of redeemable convertible preferred stock to redemption value (Preferred Stock)		$ 2,149		$ 5,225
Accretion of redeemable convertible preferred stock to redemption value									2,560		4,813		7,373
Exercises of vested stock options									10				10
Exercises of vested stock options (in shares)							1,344
Stockholders' Equity, Ending Balance at Dec. 31, 2015							$ 25		$ 219,122	$ (251)	$ (49,822)		$ 169,074
Shares, Outstanding, Ending Balance at Dec. 31, 2015							24,930,979
Issuance of preferred stock, net of discount and issuance costs (in shares)	0	20,000,000	30,000,001		30,000,001

Redeemable Convertible Preferr6

Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($)	6 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Preferred Stock
Issuance costs			$ 9,000
Redeemable Convertible Preferred Stock Series A | Preferred Stock
Convertible Notes Payable	$ 2,929,000
Tranche Rights	2,600,000
Issuance costs	$ 22,000	$ 1,000
Redeemable Convertible Preferred Stock Series B
Issuance costs		220,000
Redeemable Convertible Preferred Stock Series B | Preferred Stock
Issuance costs		220,000
Discount		$ 5,000,000

Statement of Cash Flows

Statement of Cash Flows - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Cash flow from operating activities
Net loss	$ (3,833)	$ (29,672)	$ (16,317)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense		4,027	425
Depreciation		600	184
Amortization of premiums and discounts on marketable securities		452
Change in fair value of preferred stock tranche liability		9,750	1,949
Non-cash interest on convertible promissory notes payable	67		2
Expense related to shares issued in connection with services performed			250
Deferred rent		277	(342)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets		(234)	(1,185)
Other non-current assets	(14)	14	(7)
Deferred revenue		54,982
Accounts payable	950	(942)	604
Accrued expenses	105	2,788	537
Other non-current liabiliites		(189)	186
Lease incentive benefit			1,112
Net cash provided by (used in) operating activities	(2,725)	41,299	(11,918)
Cash flow from investing activities
Purchases of property and equipment		(1,030)	(2,988)
Change in restricted cash			(314)
Purchases of marketable securities		(220,399)
Proceeds from marketable securities		26,660
Net cash used in investing activities		(194,769)	(3,302)
Cash flow from financing activities
Proceeds from issuance of Series A redeemable convertible preferred stock and tranche rights, net of issuance costs	2,860	19,999	22,040
Proceeds from issuance of Series B redeemable convertible preferred stock, net of discount and issuance costs		84,780
Proceeds from issuance of common stock and restricted stock		10	80
Conversion of preferred stock to common stock		72,955
Net cash provided by financing activities	2,860	177,744	22,120
Net increase in cash and cash equivalents	135	24,274	6,900
Cash and cash equivalents, beginning of period		7,035	135
Cash and cash equivalents, end of period	$ 135	31,309	7,035
Supplemental Disclosure of Cash and non-cash Activities
Accretion of redeemable convertible preferred stock to redemption value		7,373	1,366
Exchange of promissory notes payable and accrued interest into Series A redeemable convertible preferred stock and tranche rights		$ 150,187	$ 2,929

Nature of business

Nature of business	12 Months Ended
	Dec. 31, 2015
Nature of business
Nature of business	VOYAGER THERAPEUTICS INC. NOTES TO 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 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 fourth quarter of 2016. The Company is devoting substantially all of its efforts to product research and development, market development, and raising capital. The Company is subject to risks common to companies in the biotechnology and gene therapy industry, including but not limited to, risks of failure of 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. The Company has incurred annual net operating losses in every year since its inception. As of December 31, 2015, the Company had incurred losses since inception of $ 49.8 million. The Company has not generated any product revenues and has financed its operations primarily through public offerings and private placements of its equity securities, and funding from its collaboration with Genzyme. There can be no assurance that the Company will be able to obtain additional debt or equity financing or generate product revenue or revenues from collaborative partners, on terms acceptable to the Company, on a timely basis or at all. The failure of the Company to obtain sufficient funds on acceptable terms when needed could have a material adverse effect on the Company’s business, results of operations, and financial condition.

Summary of significant accounti

Summary of significant accounting policies	12 Months Ended
	Dec. 31, 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 consolidated financial statements include those of the Company and its subsidiary, Voyager Securities Corporation, after elimination of all intercompany accounts and transactions. The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Initial public offering On November 16, 2015 the Company completed the sale of 5,750,000 shares of its common stock in its initial public offering (the “IPO”), at a price to the public of $14.00 per share, resulting in net proceeds to the Company of $ 72.9 million after deducting underwriting discounts and commissions and 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. Upon 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. 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. 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 December 31, 2015 consist of the following: Gross Gross Amortized Unrealized Unrealized Fair Cost Gains Losses Value (in thousands) As of December 31, 2015 Money market funds included in cash and cash equivalents $ $ — $ — $ Total money market funds included in cash and cash equivalents $ $ — $ — $ Marketable securities: U.S. Treasury notes — U.S. Government agency bonds — Total marketable securities $ $ — $ $ Total money market funds and marketable securities $ $ — $ $ The estimated fair value of the Company’s marketable securities balance at December 31, 2015 , by contractual maturity, is as follows: Due in one year or less $ Due after one year through two years Total marketable securities $ Restricted Cash At December 31, 2015 and 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 December 31, 2015 . Revenue Recognition As of December 31, 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 compensatio n awards subject to performance conditions over the remaining service period when management determines that achievement of the performance condition is probable. Management evaluates when the achievement of a performance condition is probable based on the expected satisfaction of the performance conditions as of the reporting date. Income Taxes Income taxes are recorded in accordance with ASC Topic 740, Income Taxes (“ASC 740”), which provides for deferred taxes using an asset and liability approach. Under this method, deferred tax assets and liabilities are determined based on the difference between the financial reporting and the tax reporting basis of assets and liabilities and are measured using the enacted tax rates and laws that are expected to be in effect when the differences are expected to reverse. The Company provides a valuation allowance against net deferred tax assets unless, based upon the weight of available evidence, it is more likely than not that the deferred tax assets will be realized. The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. As of December 31, 2015 , the Company does not have any significant uncertain tax positions. Comprehensive Loss Comprehensive loss is comprised of net loss and other comprehensive income or loss. Other comprehensive loss consists of unrealized losses on marketable securities. Net Loss Per Share Attributable to Common Stockholders Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted ‑average number of shares of common stock outstanding during the period, without consideration for potentially dilutive securities. Diluted net loss per share attributable to common stockholders is computed by dividing the net loss attributable to common stockholders by the weighted ‑average number of shares of common stock and potentially dilutive securities outstanding for the period determined using the treasury ‑stock and if ‑converted methods. For purposes of the diluted net loss per share attributable to common stockholders calculation, redeemable convertible preferred stock, unvested restricted common stock, and outstanding stock options are considered to be potentially dilutive securities, but are excluded from the calculation of diluted net loss per share attributable to common stockholders because their effect would be anti ‑dilutive and therefore, basic and diluted net loss per share attributable to common stockholders were the same for all periods presented. The following table sets forth the outstanding potentially dilutive securities that have been excluded in the calculation of diluted net loss per share attributable to common stockholders because to do so would be anti ‑dilutive (in common stock equivalent shares): As of December 31, 2015 2014 2013 Redeemable convertible preferred stock — — Unvested restricted common stock — Outstanding stock options — — Total — All of the Company’s outstanding convertible preferred stock automatically converted into shares of common stock as of November 16, 2015, resulting in the issuance by the Company of an additional 17,647,054 shares of common stock. Concentrations of Credit Risk and Off ‑Balance Sheet Risk The Company has no financial instruments with off ‑balance sheet risk such as foreign exchange contracts, option contracts or other foreign hedging arrangements. Financial instruments that potentially subject the Company to a concentration of credit risk are cash and cash equivalents. The Company’s cash is held in accounts at a financial institution that may exceed federally insured limits. The Company has not experienced any credit losses in such accounts and does not believe it is exposed to any significant credit risk on these funds. Concentration of Suppliers The Company is dependent on a third ‑party manufacturer to supply certain products for research and development activities in its programs. In particular, the Company relies 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. The adoption of this accounting standard may affect our financial statement disclosures in future periods . In November 2015, the FASB issued ASU 2015-17, Balance Sheet Classification of Deferred Taxes, which simplifies the classification of deferred tax assets and liabilities. The new standard requires that all deferred tax assets and liabilities, along with any related valuation allowance, be classified as noncurrent on the balance sheet. The standard is effective for interim and annual periods beginning after December 15, 2016 and allows for early adoption using a full retrospective method or a prospective method. The Company adopted ASU 2015-17 as of December 31, 2015, which resulted in the netting of deferred tax assets and liabilities as non-current assets on the consolidated balance sheet. The adoption of the standard did not have a significant impact on the Company’s financial statements as the Company maintained a full valuation allowance at December 31, 2015 .

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2015
Fair Value Measurements
Fair value of financial measurements	3. Fair Value Measurements All Convertible Preferred Stock converted at the time of the IPO, therefore there were no liabilities outstanding as of December 31, 2015 . Assets and liabilities measured at fair value on a recurring basis as of December 31, 2014 are as follows: Quoted Prices Significant in Active Other Significant Markets for Observable Unobservable Identical Assets Inputs Inputs Liabilities Total (Level 1) (Level 2) (Level 3) (in thousands) Convertible PS tranche liability $ $ — $ — $ Total $ $ — $ — $ 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 (in thousands) Balance at December 31, 2014 $ Changes in fair value Reclassification to Series A Preferred Stock Balance at December 31, 2015 $ — Assets and liabilities measured at fair value on a recurring basis as of 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) (in thousands) Money market funds included in cash and cash equivalents $ $ $ — $ — Marketable securities: U.S. Treasury notes — — U.S. Government agency securities — — Total $ $ $ $ — 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 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 .

Prepaid expenses and other curr

Prepaid expenses and other current assets	12 Months Ended
	Dec. 31, 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: As of December 31, 2015 2014 (in thousands) Prepaid insurance $ $ Prepaid research and development contracts Accrued interest receivable — Other current assets Total $ $

Property and equipment, net

Property and equipment, net	12 Months Ended
	Dec. 31, 2015
Property and equipment, net
Property and equipment, net	5. Property and Equipment, net Property and equipment, net consists of the following: As of December 31, 2015 2014 (in thousands) Laboratory equipment $ $ Furniture and office equipment Leasehold improvements Less: accumulated depreciation Property and equipment, net $ $ The Company recorded $ 600,000 and $ 184,000 in depreciation expense during the years ended December 31, 2015 and 2014 , respectively. The Company did not have depreciation expense for the period from June 19, 2013 (Inception) to December 31, 2013 .

Accrued expenses

Accrued expenses	12 Months Ended
	Dec. 31, 2015
Accrued expenses
Accrued expenses	6. Accrued Expenses Accrued expenses consist of the following: As of December 31, 2015 2014 (in thousands) Patent costs $ $ Research and development costs Professional services Employee compensation costs Other Total $ $

Commitments and contingencies

Commitments and contingencies	12 Months Ended
	Dec. 31, 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 hav e been recorded as fixed assets. Rent expense of approximately $ 910,000 , $ 686,000 , and $ 48,000 was incurred during the years ended December 31, 2015 , 2014 , and the period ended December 31, 2013 , respectively. Future annual minimum lease payments at December 31, 2015 are as follows (in thousands): Total Minimum Lease Payments (in thousands) 2016 2017 2018 2019 $ In J anuary 2016, the Company executed an amendment to extend the lease to December 2024 and an agreement to lease an additional facility for approximately 26,000 square feet in Cambridge, Massachusetts . The additional facility will include laboratory and office space, and is projected to be ready for occupancy in early 2017. The table above excludes estimated payments of approximately $25.0 million, related to the amendment extending the 75 Sidney Street lease to December 2024 and the new lease for approximately 26,000 square feet in Cambridge, Massach usetts to December 2024 . 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 $ Parkinson’s Program Friedreich’s Ataxia Program Spinal Muscular Atrophy Program Future Program Committee Obligations: AJSC DAC License Option and related deliverables Total $ 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 year ended December 31, 2015 , the Company recognized $ 17,334,000 of revenue associated with its collaboration with Genzyme related to research and development services performed during the period. As of December 31, 2015 , there is $ 54,982,000 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 year ended December 31, 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, U M ass and the Company entered into a Collaboration Agreemen t wherein the Company granted UM ass 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 U M ass and MassBiologics in October 2014. On October 20, 2014, the Company entered into a Collaboration Agreement with UM ass and MassBiologics (of the U M ass Medical School). Under the agreement, the Company shall (i) fund certain projects that will be conducted by U M ass or MassBiologics, (ii) fund certain educational programs of U M ass, 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 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 2015 , the Company had provided cumulative funding of approximately $ 376,000 and $ 1,867,000 , respectively, which exceeded costs incurred by $ 376,000 and $0 , respectively . 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 December 31, 2015 . Research and development costs incurred by MassBiologics under the project agreement will be expensed by the Company as incurred. Other Agreements During 2015 and 2014 , the Company entered into various agreements with contract research organizations and institutions to license intellectual property. In consideration for the licensed rights the Company generally made upfront payments, which were recorded as research and development expense as the acquired technologies were considered in ‑process research and development. During the years ended December 31, 2015 and 2014 , the Company paid $ 75,000 and $ 830,000 , 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, 2015 and 2014 , 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, 2015 and 2014 , the Company incurred $ 295,000 and $ 872,000 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, 2015 or December 31, 2014 .

Redeemable Convertible Prefer15

Redeemable Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2015
Redeemable Convertible Preferred Stock Abstract
Redeemable Convertible Preferred Stock [Texl Block]	8. Redeemable Convertible Preferred Stock In November 2015, upon the closing of the Company’s IPO, all issued and outstanding redeemable convertible preferred stock was automatically converted into 17,647,054 shares of common stock, see Note 2. The Company has newly authorized preferred stock amounting to 5,000,000 and 0 shares as of December 31, 2015 and 2014 , respectively. The newly authorized preferred stock was classified under stockholders’ equity (deficit) at December 31, 2015. 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 45,000,000 shares of Series A Preferred Stock were issued during 2014 and 2015. These shares were issued at various closings in 2014 and 2015 for $1.00 per share. The shares were issued in exchange for cash proceeds of $42,039,000 , net of issuance costs of $32,000 , and the exchange of outstanding redeemable Convertible Notes, including accrued interest, of approximately $2,929,000 . 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) (in thousands) Tranche Right I $ Tranche Right II Total value of Tranche Rights $ 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 December 31, 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 $1,245,000 related to this issuance in the net loss attributable to common shareholders for the year ended December 31, 2015 . Series B Preferred Stock 30,000,001 shares of Series B Preferred Stock were issued during 2015 . 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. 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. Immediately prior to the IPO, 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 . 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

Common Stock	12 Months Ended
	Dec. 31, 2015
Common stock Abstract
Stockholders' Equity Note Disclosure [Text Block]	9. Common Stock As of December 31, 2015 and 2014 , the Company had authorized 120,000,000 and 65,000,000 s hares 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 December 31, 2015 2014 Shares reserved for Series A Preferred Stock outstanding — Shares reserved for future issuances of Series A Preferred Stock — Shares reserved for vesting of restricted stock awards under the Founder Agreements Shares reserved for vesting of restricted stock awards under the 2014 Option and Stock Plan Shares reserved for exercise of stock options Shares reserved for issuances under the 2015 Stock Option Plan — Shares reserved for employee stock purchase plan —

Stock-based compensation

Stock-based compensation	12 Months Ended
	Dec. 31, 2015
Stock based compensation
Disclosure of Compensation Related Costs, Share-based Payments [Text Block]	10. Stock ‑Based Compensation 2015 Stock Option Plan In October 2015, the Company ’ s board of directors and stockholders approved the 2015 Stock Option and Incentive Plan, 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. During the year ended December 31, 2015 , the Company issued 960,275 stock options to employees and directors and 69,793 stock options to non ‑employees. As of December 31, 2015 , there were 1,620,479 shares available for future issuance under the 2015 Plan. 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, August 2015 and October 2015 the Company further amended the Plan to allow for the issuance of up to 2,000,000 , 2,047,058 , 2,669,411 and 2,998,823 shares of Common Stock, respectively. During 2014 the Company issued only restricted stock awards under the Plan and during 2015 the Company only granted stock options. The terms of stock awards agreements, including vesting requirements, 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. As of December 31, 2014 and 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 2015 related to these awards. 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. 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: Total compensation cost recognized for all stock ‑based compensation awards in the statements of operations and comprehensive loss is as follows: Total compensation cost recognized for all stock ‑based compensation awards in the statements of operations and comprehensive loss is as follows: Period from June 19, 2013 Year Ended (Inception) to December 31, December 31, 2015 2014 2013 (in thousands) Research and development $ $ $ — General and administrative — Total stock-compensation expense $ $ $ — Restricted Stock A summary of the status of and changes in unvested restricted stock as of December 31, 2015 , 2014 , and 2013 was as follows: Weighted Average Grant Date Fair Value Shares Per Share Unvested restricted common stock as of December 31, 2013 — Issued $ Vested $ Repurchased — — Unvested restricted common stock as of December 31, 2014 $ Issued — Vested $ Repurchased $ Unvested restricted common stock as of December 31, 2015 $ The expense related to awards granted to employees and non ‑employees w as $ 455,000 and $ 2,551,000 , respectively, for the year ended December 31, 2015 . As of December 31, 2015 , the Company had unrecognized stock ‑based compensation expense related to its unvested restricted stock awards of $ 32,047,000 , which is expected to be recognized over the remaining weighted average vesting period of 2.35 years. Stock Options A summary of the status of, and changes in, stock options as of December 31, 2015 and 2014 was as follows: Weighted Remaining Aggregate Average Contractual Intrinsic Exercise Life Value Shares Price (in years) (in thousands) Outstanding at December 31, 2014 — Granted $ Exercised $ Cancelled or forfeited $ Outstanding at December 31, 2015 $ $ Exercisable at December 31, 2015 $ $ Vested and expected to vest at December 31, 2015 $ $ Using the Black ‑Scholes option pricing model, the weighted average fair value of options granted to employees and directors during the year ended December 31, 2015 w as $ 5.68 . T he expense related to awards granted to employees and directors was $ 720,000 for the year ended December 31, 2015 . There were no stock options granted during the year ended December 31, 2014 or the period ended December 31, 2013 . The fair value of each option issued to employees and directors was estimated at the date of grant using the Black ‑Scholes option pricing model with the following weighted ‑average assumptions: Year Ended December 31, 2015 Risk-free interest rate % Expected dividend yield — % Expected term (in years) Expected volatility % Using the Black ‑Scholes option pricin g model, the weighted average grant date fair value of options granted to non ‑employees during the year ended December 31, 2015 was $ 7.95 . Unvested o ptions granted to non ‑employees are revalued at each measurement period until they vest. The expen se related to awar ds granted to non ‑employees was $ 301,000 for the year ended December 31, 2015 . There were no stock options granted during the 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: Year Ended December 31, 2015 Risk-free interest rate % Expected dividend yield — % Expected term (in years) Expected volatility % As of December 31, 2015 , the Company had unrecognized stock ‑based compensation expense related to its unvested stock option s of $ 4,753,000 which is expe cted to be recognized over the remaining weighted average vesting period of 3.36 years .

401(k) Savings Plan

401(k) Savings Plan	12 Months Ended
	Dec. 31, 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

Related party transactions	12 Months Ended
	Dec. 31, 2015
Related party transactions
Related party transactions	1 3 . 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 $ 122,000 , $ 1,280,000 , and $ 2,379,000 d uring the years ended December 31, 2015 , 2014 , and the period ended December 31, 2013 , respectively. As of December 31, 2015 , the Company included approximately $7,000 in accounts payable related to service fees charged by this investor. During the year e nded December 31, 2015 the Company recognized $ 17,334,000 of revenue associated with its collaboration with Genzyme related to research and development services provided during this period. The company also recognized $ 2,316,000 of expense during the year ended December 31, 2015 related to in ‑kind services provided by Genzyme associated with the collaboration arrangement.

Selected quarterly financial da

Selected quarterly financial data (unaudited)	12 Months Ended
	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data	1 4 . Selected Quarterly Financial Data (Unaudited) The following table contains quarterly financial information for 2015 and 2014 . The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period. 2015 First Second Third Fourth Quarter Quarter Quarter Quarter Total (in thousands, except per share data) Collaboration revenue $ $ $ $ $ Total operating expenses Loss from operations Net loss attributable to common shareholders Net loss per share applicable to common stockholders – basic and diluted $ $ $ $ $ 2014 First Second Third Fourth Quarter Quarter Quarter Quarter Total (in thousands, except per share data) Collaboration revenue $ — $ — $ — $ — $ — Total operating expenses Loss from operations Net loss attributable to common shareholders Net loss per share applicable to common stockholders – basic and diluted $ $ $ $ $

Summary of significant accoun21

Summary of significant accounting policies (Policies)	12 Months Ended
	Dec. 31, 2015
Summary of significant accounting policies
Basis of presentation	The accompanying consolidated financial statements include those of the Company and its subsidiary, Voyager Securities Corporation, after elimination of all intercompany accounts and transactions. The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Initial public offering
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.
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.
Restricted cash	At December 31, 2015 and 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 equipement	Property and equipment consists of laboratory equipment, furniture and office equipment and leasehold improvements and is stated at cost, less accumulated depreciation. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred; while costs of major additions and betterments are capitalized. Depreciation is calculated over the estimated useful lives of the assets using the straight ‑line method.
Impairment of long-lived assets	The Company evaluates long ‑lived assets for potential impairment when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparing the book values of the assets to the expected future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book values of the assets exceed their fair value. The Company has not recognized any impairment losses from inception through December 31, 2015 .
Revenue Recognition	As of December 31, 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 compensatio n awards subject to performance conditions over the remaining service period when management determines that achievement of the performance condition is probable. Management evaluates when the achievement of a performance condition is probable based on the expected satisfaction of the performance conditions as of the reporting date.
Income taxes	Income taxes are recorded in accordance with ASC Topic 740, Income Taxes (“ASC 740”), which provides for deferred taxes using an asset and liability approach. Under this method, deferred tax assets and liabilities are determined based on the difference between the financial reporting and the tax reporting basis of assets and liabilities and are measured using the enacted tax rates and laws that are expected to be in effect when the differences are expected to reverse. The Company provides a valuation allowance against net deferred tax assets unless, based upon the weight of available evidence, it is more likely than not that the deferred tax assets will be realized. The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. As of December 31, 2015 , the Company does not have any significant uncertain tax positions.
Net loss per share	Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted ‑average number of shares of common stock outstanding during the period, without consideration for potentially dilutive securities. Diluted net loss per share attributable to common stockholders is computed by dividing the net loss attributable to common stockholders by the weighted ‑average number of shares of common stock and potentially dilutive securities outstanding for the period determined using the treasury ‑stock and if ‑converted methods. For purposes of the diluted net loss per share attributable to common stockholders calculation, redeemable convertible preferred stock, unvested restricted common stock, and outstanding stock options are considered to be potentially dilutive securities, but are excluded from the calculation of diluted net loss per share attributable to common stockholders because their effect would be anti ‑dilutive and therefore, basic and diluted net loss per share attributable to common stockholders were the same for all periods presented.
Concentration 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. The adoption of this accounting standard may affect our financial statement disclosures in future periods .

Summary of significant accoun22

Summary of significant accounting policies (Tables)	12 Months Ended
	Dec. 31, 2015
Summary of significant accounting policies
Schedule of Marketable securities	Marketable securities as of December 31, 2015 consist of the following: Gross Gross Amortized Unrealized Unrealized Fair Cost Gains Losses Value (in thousands) As of December 31, 2015 Money market funds included in cash and cash equivalents $ $ — $ — $ Total money market funds included in cash and cash equivalents $ $ — $ — $ Marketable securities: U.S. Treasury notes — U.S. Government agency bonds — Total marketable securities $ $ — $ $ Total money market funds and marketable securities $ $ — $ $
Estimated fair value of marketable securities by contractual maturity	The estimated fair value of the Company’s marketable securities balance at December 31, 2015 , by contractual maturity, is as follows: Due in one year or less $ Due after one year through two years Total marketable securities $
Outstanding potentially dilutive securities excluded in the calculation of diluted 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 attributable to common stockholders because to do so would be anti ‑dilutive (in common stock equivalent shares): As of December 31, 2015 2014 2013 Redeemable convertible preferred stock — — Unvested restricted common stock — Outstanding stock options — — Total —

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 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 $ $ — $ — $ Total $ $ — $ — $ Assets and liabilities measured at fair value on a recurring basis as of December 31 , 2015 are as follows (in thousands): Quoted Prices Significant in Active Other Significant Markets for Observable Unobservable December 31 , Identical Assets Inputs Inputs Assets 2015 (Level 1) (Level 2) (Level 3) (unaudited) Money market funds included in cash and cash equivalents $ $ $ — $ — Marketable securities: U.S. Treasury notes — — U.S. government agency securities — — Total $ $ $ $ —
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 (in thousands) Balance at December 31, 2014 $ Changes in fair value Reclassification to Series A Preferred Stock Balance at December 31, 2015 $ —

Prepaid expenses and other cu24

Prepaid expenses and other current assets (Tables)	12 Months Ended
	Dec. 31, 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: As of December 31, 2015 2014 (in thousands) Prepaid insurance $ $ Prepaid research and development contracts Accrued interest receivable — Other current assets Total $ $

Property and equipment, net (Ta

Property and equipment, net (Tables)	12 Months Ended
	Dec. 31, 2015
Property and equipment, net
Schedule of property and equipment, net	Property and equipment, net consists of the following: As of December 31, 2015 2014 (in thousands) Laboratory equipment $ $ Furniture and office equipment Leasehold improvements Less: accumulated depreciation Property and equipment, net $ $

Accrued expenses (Tables)

Accrued expenses (Tables)	12 Months Ended
	Dec. 31, 2015
Accrued expenses
Schedule of accrued expenses	Accrued expenses consist of the following: As of December 31, 2015 2014 (in thousands) Patent costs $ $ Research and development costs Professional services Employee compensation costs Other Total $ $

Commitments and contingencies (

Commitments and contingencies (Tables)	12 Months Ended
	Dec. 31, 2015
Commitments and Contingencies
Future annual minimum lease payments	Future annual minimum lease payments at December 31, 2015 are as follows (in thousands): Total Minimum Lease Payments (in thousands) 2016 2017 2018 2019 $
Allocable arrangement considateraition allocation	Unit of Accounting Amount (in thousands) Research and Development Services for: Huntington’s Program $ Parkinson’s Program Friedreich’s Ataxia Program Spinal Muscular Atrophy Program Future Program Committee Obligations: AJSC DAC License Option and related deliverables Total $

Redeemable Convertible Prefer28

Redeemable Convertible Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2015
Redeemable Convertible Preferred Stock Abstract
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) (in thousands) Tranche Right I $ Tranche Right II Total value of Tranche Rights $

Common Stock (Tables)

Common Stock (Tables)	12 Months Ended
	Dec. 31, 2015
Common stock Abstract
Schedule of shares reserved for future issuance	Shares Reserved For Future Issuance As of December 31, 2015 2014 Shares reserved for Series A Preferred Stock outstanding — Shares reserved for future issuances of Series A Preferred Stock — Shares reserved for vesting of restricted stock awards under the Founder Agreements Shares reserved for vesting of restricted stock awards under the 2014 Option and Stock Plan Shares reserved for exercise of stock options Shares reserved for issuances under the 2015 Stock Option Plan — Shares reserved for employee stock purchase plan —

Stock based compensation (Table

Stock based compensation (Tables)	12 Months Ended
	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost recognized for all stock-based compensation awards	Total compensation cost recognized for all stock ‑based compensation awards in the statements of operations and comprehensive loss is as follows: Total compensation cost recognized for all stock ‑based compensation awards in the statements of operations and comprehensive loss is as follows: Period from June 19, 2013 Year Ended (Inception) to December 31, December 31, 2015 2014 2013 (in thousands) Research and development $ $ $ — General and administrative — Total stock-compensation expense $ $ $ —
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, 2013 — Issued $ Vested $ Repurchased — — Unvested restricted common stock as of December 31, 2014 $ Issued — Vested $ Repurchased $ Unvested restricted common stock as of December 31, 2015 $
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 $ Exercised $ Cancelled or forfeited $ Outstanding at December 31, 2015 $ $ Exercisable at December 31, 2015 $ $ Vested and expected to vest at December 31, 2015 $ $
Employees and directors stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Assumptions	Year Ended December 31, 2015 Risk-free interest rate % Expected dividend yield — % Expected term (in years) Expected volatility %
Non-employees stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Assumptions	Year Ended December 31, 2015 Risk-free interest rate % Expected dividend yield — % Expected term (in years) Expected volatility %

Income taxes (Tables)

Income taxes (Tables)	12 Months Ended
	Dec. 31, 2015
Income taxes
Schedule of reconciliation of the expected income tax (benefit) computed using the federal statutory income tax rate to the effective income tax rate	A reconciliation of the U.S. federal statutory tax rate to the Company’s effective tax rate is as follows: Period from June 19, 2013 (Inception) to Year ended December 31, December 31, 2015 2014 2013 Income tax computed at federal statutory tax rate % % % State taxes, net of federal benefit % % % General business credit carryovers % % % Non-deductible expenses % % % Change in valuation allowance % % % — % — % — %
Schedule of principal components of the Company’s deferred tax assets and liabilities	The s ignificant components of the Company’s deferred tax assets and liabilities as of December 31, 2015 and 2014 are as follows: Year ended December 31, 2015 2014 (in thousands) Deferred tax assets: Net operating loss carryforwards $ $ Tax credit carryforwards Deferred rent Non-deductible expenses Intangibles Stock compensation — Total deferred tax assets Less valuation allowance Net deferred tax assets Deferred tax liabilities—depreciation and amortization Net deferred taxes $ — $ —

Selected quarterly financial 32

Selected quarterly financial data (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Information	2015 First Second Third Fourth Quarter Quarter Quarter Quarter Total (in thousands, except per share data) Collaboration revenue $ $ $ $ $ Total operating expenses Loss from operations Net loss attributable to common shareholders Net loss per share applicable to common stockholders – basic and diluted $ $ $ $ $ 2014 First Second Third Fourth Quarter Quarter Quarter Quarter Total (in thousands, except per share data) Collaboration revenue $ — $ — $ — $ — $ — Total operating expenses Loss from operations Net loss attributable to common shareholders Net loss per share applicable to common stockholders – basic and diluted $ $ $ $ $

Summary of significant accoun33

Summary of significant accounting policies - Basis of Presentation - (Details) $ / shares in Units, $ in Millions	Nov. 16, 2015USD ($)$ / sharesshares	Oct. 29, 2015	Nov. 30, 2015shares	Dec. 31, 2015shares	Dec. 31, 2014shares
Commmon stock sales				24,930,979	814,834
Reverse split ratio		4.25
Shares issued upon conversion of redeemable convertible preferred stock	17,647,054		17,647,054
IPO
Commmon stock sales	5,750,000
Price per share | $ / shares	$ 14
Estimated net proceeds | $	$ 72.9
Common Stock
Shares issued upon conversion of redeemable convertible preferred stock				17,647,054

Summary of significant accoun34

Summary of significant accounting policies - Marketable Securities - (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Investments [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 31,309,000	$ 7,035,000	$ 135,000
Total Money Market Funds And Marketable Securities Fair Value	222,637,000
Marketable securities	193,036,000
Total Money Market Funds And Marketable Securities Amortized Cost	222,888,000
Gross Unrealized Loss	251,000
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Due in one year or less	163,028,000
Due after one year through two years	30,008,000
Total marketable securities	193,036,000
Marketable Securities [Member]
Schedule of Investments [Line Items]
Marketable Securities, Amortized Cost Basis	193,287,000
Marketable securities	193,036,000
Gross Unrealized Loss	251,000
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Total marketable securities	193,036,000
Money market funds
Schedule of Investments [Line Items]
Cash and Cash Equivalents, at Carrying Value	29,601,000
Cash and Cash Equivalents, Fair Value Disclosure	29,601,000
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	158,166,000
Marketable securities	157,981,000
Gross Unrealized Loss	185,000
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Total marketable securities	157,981,000
U. S. Government agency bonds | Marketable Securities [Member]
Schedule of Investments [Line Items]
Marketable Securities, Amortized Cost Basis	35,121,000
Marketable securities	35,055,000
Gross Unrealized Loss	66,000
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity [Abstract]
Total marketable securities	$ 35,055,000

Summary of significant accoun35

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

Summary of significant accoun36

Summary of significant accounting policies - Net loss per share - (Details) - USD ($) $ in Thousands	3 Months Ended								6 Months Ended	12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share, Pro Forma [Abstract]
Antidilutive Securities Excluded from Computation of Earnings Per Share (in common stock equivalent shares)										4,723,719	8,461,169	0
Add: Changes in fair value of preferred stock tranche liability										$ 9,750	$ 1,949
Net loss attributable to common stockholders	$ (8,817)	$ (6,914)	$ (6,746)	$ (15,813)	$ (5,197)	$ (4,411)	$ (4,864)	$ (3,211)	$ (3,833)	(38,290)	(17,683)
Accretion of redeemable convertible preferred stock to redemption value										(7,373)	$ (1,366)
Add: Accrued dividends on temporary equity										$ 1,245
Weighted average number of common shares outstanding, basic and diluted (in shares)									2,352	4,191,210	635,448
Redeemable convertible preferrred stock
Earnings Per Share, Pro Forma [Abstract]
Antidilutive Securities Excluded from Computation of Earnings Per Share (in common stock equivalent shares)											5,882,352	0
Unvested restricted common stock
Earnings Per Share, Pro Forma [Abstract]
Antidilutive Securities Excluded from Computation of Earnings Per Share (in common stock equivalent shares)										1,818,261	2,578,817	0
Outstanding stock options
Earnings Per Share, Pro Forma [Abstract]
Antidilutive Securities Excluded from Computation of Earnings Per Share (in common stock equivalent shares)										2,905,458		0

Fair Value Measurements - Asset

Fair Value Measurements - Assets measured on a recurring basis on year end - (Details) $ in Thousands	Dec. 31, 2015USD ($)
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 - Serie

Fair Value Measurements - Series A Preferred Stock - (Details) - Series A Preferred Stock Tranche Liability - Probability Weighted Present Value Model $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2014USD ($)$ / 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 - Ass39

Fair Value Measurements - Assets measured on a recurring basis - (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015USD ($)
Carrying Value
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	$ 222,637
Level 1 | Estimate of Fair Value
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	187,582
Level 2 | Estimate of Fair Value
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	35,055
Level 3
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance, begining	6,305
Changes in fair value	9,750
Reclassification to Series A Preferred Stock	(16,055)
U.S. Treasury notes | Carrying Value
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	157,981
U.S. Treasury notes | Level 1 | Estimate of Fair Value
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	157,981
U. S. Government agency bonds | Carrying Value
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	35,055
U. S. Government agency bonds | Level 2 | Estimate of Fair Value
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	35,055
Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	29,601
Money market funds | Carrying Value
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	29,601
Money market funds | Level 1 | Estimate of Fair Value
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 29,601

Prepaid expenses and other cu40

Prepaid expenses and other current assets (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Prepaid expense and other current assets
Prepaid expenses	$ 424	$ 18
Prepaid research and development contracts	340	899
Accrued interest receivable	411
Other current assets	382	406
Total	$ 1,557	$ 1,323

Property and equipment, net (De

Property and equipment, net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 4,018	$ 2,988
Less: accumulated depreciation	(784)	(184)
Property and equipment, net	3,234	2,804
Depreciation expense	600	184
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	2,183	1,223
Furniture and office equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	499	441
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 1,336	$ 1,324

Accrued expenses (Details)

Accrued expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Accrued expenses
Patent costs	$ 235	$ 274
Research and development costs	1,329	125
Professional services	350	81
Employee compensation costs	1,338	85
Other	178	77
Total	$ 3,430	$ 642

Commitments and contingencies -

Commitments and contingencies - Operating Leases - (Details)	1 Months Ended		12 Months Ended
	Jan. 31, 2016USD ($)ft²	Mar. 31, 2014USD ($)item	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)
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			$ 686,000	$ 910,000	$ 48,000
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2,016			1,170,000
2,017			1,192,000
2,018			1,214,000
2,019			1,184,000
Total minimum lease payments			$ 4,760,000
Area of additional facility leased | ft²	26,000
Estimated lease amendment related payment	$ 25,000,000

Commitments and contingencies44

Commitments and contingencies - Significant Agreements - (Details) - Collaborative Arrangement $ in Millions	1 Months Ended
	Feb. 28, 2015USD ($)shares
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment received	$ 65
Premium over fair value on temporary equity issued	5
Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment received	65
Amount of reduction in milestone or royalty payments to be made by collaborative partner for each program specified	10
Per Milestone, maximum milestone payments to be received from collaborative partner	645
Regulatory approval milestone | Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner	265
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
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
Commerical milestone | Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner	380
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
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
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
Option exercise payment required from collaborative partner for each progam specified	30
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
Redeemable Convertible Preferred Stock Series B
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment received	30
Fair value of temporary equity issued	$ 25
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
Premium over fair value on temporary equity issued	$ 5

Commitments and contingencies45

Commitments and contingencies - Accounting Analysis - (Details) - USD ($)	1 Months Ended	3 Months Ended				12 Months Ended
	Feb. 28, 2015	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 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		$ 4,937,000	$ 4,937,000	$ 4,884,000	$ 2,576,000	$ 17,334,000
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						17,334,000
Deferred Revenue		$ 54,982,000				$ 54,982,000
Redeemable Convertible Preferred Stock Series B | Collaborative Arrangement
Allocable arrangement consideration
Non-refundable upfront payment received	30,000,000
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 contingencies46

Commitments and contingencies - UMass and MassBiologics Collaboration - (Details) - USD ($)	1 Months Ended		6 Months Ended	12 Months Ended
	Nov. 30, 2014	Jan. 31, 2014	Dec. 31, 2013	Dec. 31, 2014
Common Stock
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Issuance of common stock (in shares)			2,352
University Of Massachusetts (UMass) Collaboration Agreement | Common Stock
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				$ 376,000
Cumulative funding in excess of costs incurred for research and devlopment services performed by collaborative partner				$ 376,000

Commitments and contingencies47

Commitments and contingencies - Other Agreements - (Details)	12 Months Ended
	Dec. 31, 2015USD ($)item	Dec. 31, 2014USD ($)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	$ 872,000	$ 295,000
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	$ 830,000	$ 75,000

Redeemable Convertible Prefer48

Redeemable Convertible Preferred Stock - Series A Preferred Stock- Series A Preferred Stock- (Details) - USD ($)	Nov. 16, 2015	Nov. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Temporary Equity [Line Items]
Shares issued upon conversion of redeemable convertible preferred stock	17,647,054	17,647,054
Redeemable convertible preferred stock, outstanding shares				25,000,000
Exchange of outstanding convertible notes and accrued interest into Series A redeemable convertible preferred stock and tranche rights			$ 150,187,000	$ 2,929,000
Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable convertible referred stock, authorized shares			0	45,000,000
Redeemable convertible preferred stock, issued (in shares)			0	25,000,000
Redeemable convertible preferred stock, outstanding shares			0
Redeemable convertible preferred stock, aggregate liquidation preference (in dollars)				$ 26,086
Redeemable Convertible Preferred Stock Series A
Temporary Equity [Line Items]
Preferred Stock, Shares Authorized			45,000,000	45,000,000
Price per share			$ 1	$ 1
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 Series B
Temporary Equity [Line Items]
Redeemable convertible referred stock, authorized shares			30,000,001
Redeemable convertible preferred stock, issued (in shares)			30,000,001
Price per share			$ 3
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 Preferred Stock
Temporary Equity [Line Items]
Preferred Stock, Shares Authorized			5,000,000	0

Redeemable Convertible Prefer49

Redeemable Convertible Preferred Stock - Tranche Rights Issued with Series A Preferred Stock - (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	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	1,949,000
Accrued dividends on temporary equity	1,245,000
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
Redeemable Convertible Preferred Stock Series A
Temporary Equity [Line Items]
Price per share	$ 1	$ 1
Fair value of Tranche Rights
Total Fair value of Tranche Right Asset (Liability)	$ (2,600,000)
Proceeds from issuance of stock, net of issuance costs	$ 32,000
Redeemable convertible preferred stock, initial carrying amount		$ 21,979,000
Redeemable Convertible Preferred Stock Series A | Tranche Right I
Temporary Equity [Line Items]
Number of additional shares of temporary equity contingently obligated to be purchased by temporary equity holders	18,500,000
Price per share	$ 1
Fair value of Tranche Rights
Fair value of Tranche Right Asset	$ 1,495,000
Issuance of stock (in shares)		18,500,000
Redeemable Convertible Preferred Stock Series A | 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)
Redeemable convertible preferred stock, initial carrying amount	36,054,000
Accrued dividends on temporary equity	$ 1,245,000
Redeemable Convertible Preferred Stock Series B
Temporary Equity [Line Items]
Price per share	$ 3
Fair value of Tranche Rights
Proceeds from issuance of stock, net of issuance costs	$ 89,780,000

Redeemable Convertible Prefer50

Redeemable Convertible Preferred Stock - Series B Preferred Stock - (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Temporary Equity [Line Items]
Redeemable convertible preferred stock, outstanding shares		25,000,000
Exchange of outstanding convertible notes and accrued interest into Series A redeemable convertible preferred stock and tranche rights	$ 150,187,000	$ 2,929,000
Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable convertible referred stock, authorized shares	0	45,000,000
Redeemable convertible preferred stock, issued (in shares)	0	25,000,000
Redeemable convertible preferred stock, outstanding shares	0
Redeemable convertible preferred stock, aggregate liquidation preference (in dollars)		$ 26,086
Redeemable Convertible Preferred Stock Series B
Temporary Equity [Line Items]
Redeemable convertible referred stock, authorized shares	30,000,001
Redeemable convertible preferred stock, issued (in shares)	30,000,001
Price per share	$ 3
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 Prefer51

Redeemable Convertible Preferred Stock - Conversion - (Details)	12 Months Ended
	Dec. 31, 2015USD ($)directorVote$ / shares
Temporary Equity [Line Items]
Minimum gross proceeds from IPO that triggers conversion of temporary equity | $	$ 50
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%
Number of votes for each equivalent common share of temporary equity | Vote	1
Number of directors temporary equity holders may elect | director	2
Redeemable Convertible Preferred Stock Series A
Temporary Equity [Line Items]
Temporary equity conversion price (in dollars per share)	$ 4.25
Temporary equity liquidation preference (in dollars per share)	1
Minimum aggregate liquidation preference payments to temporary equity holders that trigger specified alternate preference payout (in dollars per share)	2.50
Redeemable Convertible Preferred Stock Series B
Temporary Equity [Line Items]
Temporary equity conversion price (in dollars per share)	12.75
Temporary equity redemption price (in dollars per share)	$ 3

Common Stock (Details)

Common Stock (Details)	12 Months Ended
	Dec. 31, 2015Vote$ / sharesshares	Dec. 31, 2014$ / sharesshares
Class of Stock [Line Items]
Common Stock, Shares Authorized	120,000,000	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)	4,723,719	13,458,104
Preferred Stock outstanding
Class of Stock [Line Items]
Shares available for future issuance (in shares)		5,882,352
Preferred Stock future issuance
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)	853,680	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)	964,581	1,510,434
Exercise of stock options
Class of Stock [Line Items]
Shares available for future issuance (in shares)	1,022,617	291,052
Issuance under share based compensation plans
Class of Stock [Line Items]
Shares available for future issuance (in shares)	262,362
Issuance under share based compensation plans | 2015 Stock Option Plan
Class of Stock [Line Items]
Shares available for future issuance (in shares)	1,620,479

Stock based compensation - 2014

Stock based compensation - 2014 Stock Option and Grant Plan - (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Oct. 31, 2015	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)		4,723,719	13,458,104
Stock-based compensation		$ 4,027,000	$ 425,000
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			2,998,823	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)		1,620,479
Unvested restricted common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock granted (in shares)			2,897,181
Restricted stock granted, original issued price (in dollars per share)			$ 0.77
Employees restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation		$ 455,000
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		$ 2,551,000
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)		1,030,068	0
Employees and directors stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation		$ 720,000
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)		960,275
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		$ 301,000
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)		69,793
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 - Foun

Stock based compensation - Founder Awards - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation	$ 4,027	$ 425
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation	3,218	297
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation	$ 809	$ 128

Stock based compensation - Rest

Stock based compensation - Restricted Stock - (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Restricted stock disclosures
Stock-based compensation	$ 4,027,000	$ 425,000
Unvested restricted common stock
Shares
Balance, beginning (in shares)	2,578,817
Issued (in shares)		2,897,181
Vested (in shares)	(717,747)	(318,364)
Repurchased (in shares)	(42,809)
Balance, ending (in shares)	1,818,261	2,578,817
Weighted Average Grant Date Fair Value Per Share
Balance, beginning (in dollars per share)	$ 0.77
Issued (in dollars per share)		$ 0.77
Vested (in dolalrs per share)	0.77	0.68
Repurchased (in dollars per share)	0.68
Balance, ending (in dollars per share)	$ 0.76	$ 0.77
Restricted stock disclosures
Unrecognized stock-based compensation expense	$ 32,047,000
Remaining weighted-average remaining vesting period	2 years 4 months 6 days
Employees restricted stock
Restricted stock disclosures
Stock-based compensation	$ 455,000
Non-employee consultants restricted stock
Restricted stock disclosures
Stock-based compensation	$ 2,551,000

Stock based compensation - Stoc

Stock based compensation - Stock Options - (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Options disclosures
Stock-based compensation	$ 4,027,000	$ 425,000
Stock options, all inclusive
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Granted (in shares)	1,030,068	0
Exercised (in shares)	(1,344)
Cancelled or forfeited (in shares)	(6,107)
Outstanding, ending balance (in shares)	1,022,617
Exerciseable (in shares)	94,762
Vested and expected to vest (in shares)	1,022,617
Weighted Average Exercise Price
Granted (in dollars per share)	$ 8.34
Exercised (in dollars per share)	8.39
Cancelled or forfeited (in dollars per share)	7.27
Outstanding (in dollars per share)	8.35
Exerciseable (in dollars per share)	7.51
Vested and expected to vest (in dollars per share)	$ 8.35
Remaining Contractual Life
Outstanding	9 years 6 months
Exerciseable	9 years 2 months 12 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Remaining Contractual Term	9 years 6 months
Aggregate Instrinsic Value
Oustanding	$ 13,887,000
Exerciseable	1,363,000
Vested and expected to vest	$ 13,887,000
Employees and directors stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 5.68
Stock-based compensation	$ 720,000
Non-employees stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 7.95
Stock-based compensation	$ 301,000

Stock based compensation - Blac

Stock based compensation - Black-Scholes options pricing model - (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Options disclosures
Stock-based compensation	$ 4,027,000	$ 425,000
Stock options, all inclusive
Options disclosures
Stock options granted (in shares)	1,030,068	0
Unrecognized stock-based compensation expense	$ 4,753,000
Remaining weighted-average remaining vesting period	3 years 4 months 10 days
Employees and directors stock options
Fair value assumptions
Risk-free interest rate	1.60%
Expected term	6 years
Expected volatility	78.60%
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 5.68
Stock-based compensation	$ 720,000
Non-employees stock options
Fair value assumptions
Risk-free interest rate	2.00%
Expected term	10 years
Expected volatility	84.00%
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 7.95
Stock-based compensation	$ 301,000

Income taxes (Details)

Income taxes (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax computed at federal statutory tax rate	34.00%	34.00%	34.00%
State taxes, net of federal benefit	6.30%	4.00%	5.50%
General business credit carryovers	2.40%	3.20%	2.20%
Non‑deductible expenses	(0.70%)	(15.60%)	(5.00%)
Change in valuation allowance	(42.00%)	(25.60%)	(36.70%)
Deferred tax assets:
Net operating loss carryforwards		$ 12,521	$ 6,643
Tax credit carryforwards		1,969	673
Deferred rent		516	626
Non‑deductible expenses		577	79
Intangibles		376	327
Stock Compensation		115
Total deferred tax assets		16,074	8,348
Less valuation allowance		(15,207)	(7,599)
Net deferred tax assets		867	749
Deferred tax liabilities—depreciation and amortization		(867)	(749)
Net deferred taxes		$ 0	$ 0

Income taxes - Operating loss c

Income taxes - Operating loss carryforwards - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 12,521	$ 6,643
Valuation Allowance	15,207	7,599
Change in the valuation allowance	7,600
Unrecognized tax benefits	0	$ 0
Federal | Tax Year 2028
Operating Loss Carryforwards [Line Items]
Research and development tax credit carryforwards	1,400
Federal | Tax Year 2033
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	32,100
State | Tax Year 2028
Operating Loss Carryforwards [Line Items]
Research and development tax credit carryforwards	900
State | Tax Year 2033
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 30,200

Related party transactions (Det

Related party transactions (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Related Party Transaction [Line Items]
Fair value of common stock issued as partial compensation for consulting and management services			$ 252,000
Investor
Related Party Transaction [Line Items]
Total amount of services received		$ 122,000		$ 2,379,000
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			$ 1,280,000
Accounts payable to related party		7,000
Genzyme | Collaboration arrangement
Related Party Transaction [Line Items]
Revenue recognized		17,334,000
Expense recognized		$ 2,316,000

Selected quarterly financial 61

Selected quarterly financial data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								6 Months Ended	12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 4,937	$ 4,937	$ 4,884	$ 2,576						$ 17,334
Total operating expenses	12,377	8,956	8,851	7,404	$ 4,496	$ 3,656	$ 3,230	$ 2,985	$ 3,766	37,588	$ 14,367
Loss from operations	(7,440)	(4,019)	(3,967)	(4,828)	(4,496)	(3,656)	(3,230)	(2,985)	(3,766)	(20,254)	(14,367)
Net loss attributable to common stockholders	$ (8,817)	$ (6,914)	$ (6,746)	$ (15,813)	$ (5,197)	$ (4,411)	$ (4,864)	$ (3,211)	$ (3,833)	$ (38,290)	$ (17,683)
Net loss per share attributable to common stock, basic and diluted	$ (0.67)	$ (5.25)	$ (5.80)	$ (15.81)	$ (6.58)	$ (6.45)	$ (8.11)	$ (6.93)	$ (1,629.68)	$ (9.14)	$ (27.83)

Uncategorized Items - vygr-2015

Label	Element	Value
Retained Earnings [Member]
Net Income (Loss) Attributable to Parent	us-gaap_NetIncomeLoss	$ (3,833,000)