Voyager Therapeutics (VYGR) 10-Q 7 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Nov. 02, 2018
Document and Entity Information
Entity Registrant Name	Voyager Therapeutics, Inc.
Entity Central Index Key	1,640,266
Document Type	10-Q
Document Period End Date	Sep. 30, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		32,556,874
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 42,142	$ 31,530
Marketable securities, current	137,459	137,522
Prepaid expenses and other current assets	2,865	2,738
Total current assets	182,466	171,790
Property and equipment, net	12,527	10,283
Deposits and other non-current assets	1,271	1,304
Marketable securities, non-current	800	1,100
Total assets	197,064	184,477
Current liabilities:
Accounts payable	1,422	1,020
Accrued expenses	8,355	11,497
Deferred revenue, current portion	20,250	3,380
Total current liabilities	30,027	15,897
Deferred rent	5,642	5,337
Deferred revenue, net of current portion	94,804	28,180
Other non-current liabilities	1,003	1,012
Total liabilities	131,476	50,426
Commitments and contingencies (see note 6)
Stockholders’ equity:
Preferred stock $0.001 par value: 5,000,000 shares authorized at September 30, 2018 and December 31, 2017; no shares issued and outstanding at September 30, 2018 and December 31, 2017
Common stock, $0.001 par value: 120,000,000 shares authorized at September 30, 2018 and December 31, 2017; 32,309,700 and 31,572,044 shares issued and outstanding at September 30, 2018 and December 31, 2017, respectively	32	32
Additional paid-in capital	312,250	295,019
Accumulated other comprehensive loss	(174)	(287)
Accumulated deficit	(246,520)	(160,713)
Total stockholders’ equity	65,588	134,051
Total liabilities and stockholders’ equity	$ 197,064	$ 184,477

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2018	Dec. 31, 2017
Condensed Consolidated Balance Sheets
Preferred Stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred Stock, Shares Authorized	5,000,000	5,000,000
Preferred Stock, Shares Issued	0	0
Preferred Stock, Shares Outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common Stock, Shares Authorized	120,000,000	120,000,000
Common Stock, Shares, Issued	32,309,700	31,572,044
Common Stock, Shares, Outstanding	32,309,700	31,572,044

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Condensed Consolidated Statements of Operations and Comprehensive Loss
Collaboration revenue	$ 2,094	$ 1,148	$ 5,611	$ 3,790
Operating expenses:
Research and development	16,632	19,561	47,991	48,933
General and administrative	6,609	4,942	25,554	14,372
Total operating expenses	23,241	24,503	73,545	63,305
Operating loss	(21,147)	(23,355)	(67,934)	(59,515)
Other income
Interest income, net	942	291	2,399	799
Other expense, net	(84)	(282)	(402)	(184)
Total other income	858	9	1,997	615
Loss before income taxes	(20,289)	(23,346)	(65,937)	(58,900)
Income tax benefit			180	31
Net loss	(20,289)	(23,346)	(65,757)	(58,869)
Other comprehensive (loss) income
Unrealized (loss) gain on available-for-sale-securities, net of income tax benefit of $53 for the three months ended September 30, 2017	(49)	(485)	108	(403)
Total other comprehensive (loss) income	(49)	(485)	108	(403)
Comprehensive loss	$ (20,338)	$ (23,831)	$ (65,649)	$ (59,272)
Net loss per share, basic and diluted	$ (0.63)	$ (0.89)	$ (2.06)	$ (2.27)
Weighted-average common shares outstanding, basic and diluted	32,191,475	26,164,527	31,977,670	25,968,849

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) $ in Thousands	3 Months Ended
	Sep. 30, 2017USD ($)
Condensed Consolidated Statements of Operations and Comprehensive Loss
Tax benefit	$ 53

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flow from operating activities
Net loss	$ (65,757)	$ (58,869)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Stock-based compensation expense	13,072	6,945
Depreciation	1,538	1,159
Amortization of premiums and discounts on marketable securities	(1,563)	197
In-kind research and development expenses	176	113
Other non-cash items	497	139
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(127)	1,764
Other non-current assets	(180)
Deferred revenue	63,388	(3,790)
Accounts payable	402	361
Accrued expenses	(3,306)	3,950
Other non-current liabilities		1,000
Lease incentive benefit	321	515
Net cash provided by (used in) operating activities	8,461	(46,516)
Cash flow from investing activities
Purchases of property and equipment	(3,782)	(3,824)
Purchases of marketable securities	(274,381)	(49,856)
Proceeds from maturities of marketable securities	276,000	110,100
Net cash (used in) provided by investing activities	(2,163)	56,420
Cash flow from financing activities
Proceeds from the exercise of stock options	3,472	1,656
Proceeds from the purchase of common stock under ESPP	842
Net cash provided by financing activities	4,314	1,656
Net increase in cash, cash equivalents, and restricted cash	10,612	11,560
Cash, cash equivalents, and restricted cash beginning of period	32,265	37,376
Cash, cash equivalents, and restricted cash end of period	42,877	$ 48,936
Supplemental disclosure of cash and non-cash activities
Impact of adopting new accounting standards	$ 20,050

Nature of business

Nature of business	9 Months Ended
	Sep. 30, 2018
Nature of business
Nature of business	VOYAGER THERAPEUTICS, INC. NOTE 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 neurological diseases. The Company is focused on neurological diseases where it believes an adeno-associated virus (“AAV”) gene therapy approach that either increases or decreases the production of a specific protein can slow or reduce the symptoms experienced by patients, and therefore have a clinically meaningful impact. The Company has built a gene therapy platform that it believes positions itself to be a leading company at the intersection of AAV gene therapy and severe neurological disease. The Company’s gene therapy platform enables it to engineer, optimize, manufacture and deliver its AAV-based gene therapies that have the potential to provide durable efficacy following a single administration. Additionally, the Company is working to identify novel AAV capsids, which are the outer viral protein shells that enclose the genetic material of the virus payload. The Company’s team of experts in the fields of AAV gene therapy and neuroscience first identifies and selects severe neurological diseases that are well-suited for treatment using AAV gene therapy. The Company then engineers and optimizes AAV vectors for delivery of the virus payload to the targeted tissue or cells. The Company’s manufacturing process employs an established system that it believes will enable production of high quality AAV vectors at commercial-scale. I n addition to the Company’s capsid optimization efforts, it leverages novel delivery paradigms, established routes of administration, and advances in dosing techniques to optimize delivery of its AAV gene therapies to target tissues, regions and cell types that are critical to the disease of interest. The Company can achieve this directly, with targeted infusions to discrete regions of the brain or spinal cord, or systemically, in conjunction with its novel capsids. The Company’s business strategy focuses on discovering, developing, manufacturing and commercializing its gene therapy programs. As part of this strategy, the Company has developed core competencies specific to AAV gene therapy development and manufacturing and is beginning to build its commercial infrastructure. This business strategy also includes business development activities that may include in-licensing activities or partnering certain programs in certain geographies with collaborators, as the Company has demonstrated through its collaboration with Sanofi Genzyme (the “ Sanofi Genzyme Collaboration”) and its collaboration with AbbVie Biotechnology Ltd (the “ AbbVie Collaboration”) . The Company is devoting substantially all of its efforts to product research and development, market development, and raising capital. The Company is subject to risks common to companies in the biotechnology and gene therapy industry, including but not limited to, risks of failure of preclinical studies and clinical trials, the need to obtain marketing approval for its product candidates, the need to successfully commercialize and gain market acceptance of its product candidates, dependence on key personnel, protection of proprietary technology, compliance with government regulations, development by competitors of technological innovations, and ability to transition from pilot-scale manufacturing to large-scale production of products. The Company has incurred annual net operating losses in every year since inception. As of September 30, 2018, the Company had an accumulated deficit of $246.5 million. The Company has not generated any product revenue and has financed its operations primarily through public offerings and private placements of its equity securities and funding from the Sanofi Genzyme Collaboration and the AbbVie Collaboration . There can be no assurance that the Company will be able to obtain additional debt or equity financing or generate product revenue or revenue from collaborative partners on terms acceptable to the Company, on a timely basis or at all. The failure of the Company to obtain sufficient funds on acceptable terms when needed could have a material adverse effect on the Company’s business, results of operations, and financial condition. Through September 30, 2018, the Company has raised approximately $405.0 million of proceeds from sales of convertible preferred stock and common stock, including its initial public offering (the “IPO”) and follow-on public offering, and from collaboration agreements. The Company believes that its cash, cash equivalents, and marketable debt securities of $179.6 million as of September 30, 2018 is sufficient to fund its current operating plan into early 2020. There can be no assurance, however, that the current operating plan will be achieved in the timeframe anticipated by the Company, or that its cash resources will fund the Company’s operating plan for the period anticipated by the Company, or if the Company needs additional funding, that such funding will be available on terms acceptable to the Company, or at all.

Summary of significant accounti

Summary of significant accounting policies	9 Months Ended
	Sep. 30, 2018
Summary of significant accounting policies
Summary of significant accounting policies	2. Summary of significant accounting policies and basis of presentation Basis of Presentation The accompanying unaudited condensed consolidated financial statements of the Company have been prepared in accordance with accounting principles generally accepted in the United States (“GAAP”) for interim financial reporting and as required by Regulation S-X, Rule 10-01. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. For further information, refer to the consolidated financial statements and footnotes included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017 as filed with the Securities and Exchange Commission (“SEC”). These interim condensed consolidated financial statements, in the opinion of management, reflect all normal recurring adjustments necessary for a fair presentation of the Company’s financial position and results of operations for the periods presented. Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”). Principles of Consolidation The unaudited interim consolidated financial statements include the accounts of the Company and its wholly owned subsidiary as disclosed in Note 2, Summary of Significant Accounting Policies, within the “Notes to Consolidated Financial Statements” accompanying its Annual Report on Form 10-K for the fiscal year ended December 31, 2017. Intercompany accounts and transactions have been eliminated. 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, 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. Revenue Recognition The Company enters into collaboration agreements which are within the scope of ASC 606, Revenue from Contracts with Customers (“ASC 606”), under which the Company licenses rights to certain of the Company’s product candidates and performs research and development services. The terms of these arrangements typically include payment of one or more of the following: non-refundable, upfront fees; reimbursement of research and development costs; development, regulatory, and commercial milestone payments; and royalties on net sales of licensed products. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine the appropriate amount of revenue to be recognized for arrangements determined to be within the scope of ASC 606, the Company performs the following five steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect consideration it is entitled to in exchange for the goods or services it transfers to the customer. The promised goods or services in the Company’s arrangement typically consist of license rights to the Company’s intellectual property and research and development services. The Company provides options to additional items in the contracts, which are accounted for as separate contracts when the customer elects to exercise such options, unless the option provides a material right to the customer. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the outset of the arrangement. Performance obligations are promised goods or services in a contract to transfer a distinct good or service to the customer and are considered distinct when (i) the customer can benefit from the good or service on its own or together with other readily available resources and (ii) the promised good or service is separately identifiable from other promises in the contract. In assessing whether promised goods or services are distinct, the Company considers factors such as the stage of development of the underlying intellectual property, the capabilities of the customer to develop the intellectual property on its own or whether the required expertise is readily available and whether the goods or services are integral or dependent to other goods or services in the contract. The Company estimates the transaction price based on the amount expected to be received for transferring the promised goods or services in the contract. The consideration may include fixed consideration or variable consideration. At the inception of each arrangement that includes variable consideration, the Company evaluates the amount of potential payments and the likelihood that the payments will be received. The Company utilizes either the most likely amount method or expected amount method to estimate the amount expected to be received based on which method best predicts the amount expected to be received. The amount of variable consideration which is included in the transaction price may be constrained, and is included in the transaction price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized will not occur in a future period. The Company’s contracts often include development and regulatory milestone payments which are assessed under the most likely amount method and constrained if it is probable that a significant revenue reversal would occur. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At the end of each reporting period, the Company re-evaluates the probability of achievement of such development milestones and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect collaboration revenues in the period of adjustment. To date, the Company has not recognized any consideration related to the achievement of development, regulatory, or commercial milestone revenue resulting from any of the Company’s collaboration arrangements. For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any consideration related to sales-based royalty revenue resulting from any of the Company’s collaboration arrangements. The Company allocates the transaction price based on the estimated stand-alone selling price of each of the performance obligations. The Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company utilizes key assumptions to determine the stand-alone selling price for service obligations, which may include other comparable transactions, pricing considered in negotiating the transaction and the estimated costs. Additionally, in determining the standalone selling price for material rights, the Company utilizes comparable transactions, clinical trial success probabilities, and estimates of option exercise likelihood. Variable consideration is allocated specifically to one or more performance obligations in a contract when the terms of the variable consideration relate to the satisfaction of the performance obligation and the resulting amounts allocated are consistent with the amounts the Company would expect to receive for the satisfaction of each performance obligation. The consideration allocated to each performance obligation is recognized as revenue when control is transferred for the related goods or services. For performance obligations which consist of licenses and other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Upfront payments and fees are recorded as deferred revenue upon receipt or when due until the Company performs its obligations under these arrangements. Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional. Recently Adopted Accounting Pronouncements In January 2016, the FASB issued ASC Update No. 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities (“Update No. 2016-01”). The purpose of Update No. 2016-01 is to improve financial reporting for financial instruments by reducing the number of items recorded to other comprehensive income. The Company adopted Update No. 2016-01 in the first quarter of 2018, using the modified retrospective method. Unrealized gains and losses previously recorded to other comprehensive income (loss) were reclassified to accumulated deficit and all future fair value changes will be recorded to other income (loss). The adoption of the standard on January 1, 2018 did not have a material impact on the Company’s consolidated financial statements. In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (“ASU 2016-15”), which simplifies certain elements of cash flow classification and is intended to reduce diversity in practice in how certain transactions are classified in the statement of cash flows. The update is effective for fiscal years, and interim periods within those years, beginning after December 15, 2017. The Company adopted ASU 2016-15 on January 1, 2018, and such adoption did not have a material impact on the Company's consolidated financial statements. In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash (“ASU 2016-18”). The amendments in ASU 2016-18 require an entity to reconcile and explain the period-over-period change in total cash, cash equivalents and restricted cash within its statements of cash flows and was applied utilizing a full retrospective approach. The Company adopted the new standard on January 1, 2018. The Company has included the necessary reconciliation within Note 4 “Cash, cash equivalents, and available-for-sale marketable securities.” In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (“ASU 2014-09”), which supersedes the revenue recognition requirements in ASC 605, Revenue Recognition (“ASC 605”), and most industry-specific guidance. The new standard requires that an entity recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The update also requires additional disclosure about the nature, amount, timing, and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. Thereafter, a series of clarifying ASUs, narrow scope improvements and practical expedients were issued. This collective guidance resulted in the new revenue standard, ASC 606. The Company adopted the new standard effective January 1, 2018 using the modified retrospective approach. The Company had one open contract, relating to the Sanofi Genzyme Collaboration, on the adoption date and has assessed it under the new revenue standard. The adoption of ASC 606 resulted in the changes to (i) the allocation of arrangement consideration, including the determination of estimated selling price and the allocation of variable consideration to specific performance obligations and (ii) the application of proportional performance as a measure of progress on service related deliverables. The Company has accounted for the impact of adopting ASC 606 as a cumulative catch-up under the modified retrospective approach, which is represented as an increase of $20.0 million to deferred revenue with an offset to accumulated deficit, effective January 1, 2018. The following financial statement line items have been shown to reflect comparative balances under ASC 606 and ASC 605 for each of the three-month period and nine-month period ended September 30, 2018, for both of the Sanofi Genzyme Collaboration and AbbVie Collaboration, collectively. Condensed Consolidated Statements of Operations and Comprehensive Loss Three months ended September 30, 2018 Under ASC 606 Under ASC 605 Effect of change (in thousands, except per share data) Collaboration revenue $ 2,094 $ 3,054 $ (960) Loss before income taxes (20,289) (19,329) (960) Net loss (20,289) (19,329) (960) Net loss per share, basic and diluted (0.63) (0.60) (0.03) Nine months ended September 30, 2018 Under ASC 606 Under ASC 605 Effect of change (in thousands, except per share data) Collaboration revenue $ 5,611 $ 7,267 $ (1,656) Loss before income taxes (65,937) (64,281) (1,656) Net loss (65,757) (64,101) (1,656) Net loss per share, basic and diluted (2.06) (2.00) (0.06) Condensed Consolidated Balance Sheets As of September 30, 2018 Under ASC 606 Under ASC 605 Effect of change (in thousands) Deferred revenue, current $ 20,250 $ 19,078 $ 1,172 Deferred revenue, non-current 94,804 74,390 20,414 Accumulated deficit (246,520) (224,934) (21,586) Condensed Consolidated Statements of Cash Flows Nine months ended September 30, 2018 Under ASC 606 Under ASC 605 Effect of change (in thousands) Net loss $ (65,757) $ (64,101) $ (1,656) Adjustments to reconcile net loss to net cash provided by operating activities: Deferred revenue 63,388 61,732 1,656 Recent Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, a comprehensive new lease accounting standard, which provides revised guidance on accounting for lease arrangements by both lessors and lessees and requires lessees to recognize a lease liability and a right-of-use asset for most leases. In July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842), Targeted Improvements , which provides an additional transition method that allows entities to initially apply the new lease requirements at the adoption date, not the earliest period presented, and recognize a cumulative effect adjustment to the opening balance of retained earnings in the period of adoption. The Company expects to elect this transition method at the adoption date of January 1, 2019. The Company also expects to elect a package of practical expedients, under which an entity need not reassess whether any expired or existing contracts are or contain leases, the lease classification for any expired or existing leases, or initial direct costs for any existing leases. The Company is currently in the process of evaluating the impact that the adoption of this guidance will have on its consolidated financial statements and related disclosures. On December 22, 2017, the SEC staff issued Staff Accounting Bulletin No. 118, to address the application of GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the Tax Cuts and Jobs Act (the “Tax Reform Act”). The Company has recognized the provisional tax impacts related to the revaluation of the deferred tax assets and liabilities and included these amounts in its consolidated financial statements for the year ended December 31, 2017. The ultimate impact may differ from these provisional amounts due to, among other things, additional analysis, changes in interpretations and assumptions the Company has made, additional regulatory guidance that may be issued, and actions the Company may take as a result of the Tax Reform Act. The accounting was completed when the Company’s 2017 U.S. corporate income tax return was filed in 2018, and no material differences arose as compared to provisional amounts initially reflected in the consolidated financial statements for the year ended December 31, 2017. In June 2018, the FASB issued ASU No. 2018-07, Compensation-Stock Compensation: Improvements to Nonemployee Share-Based Payment Accounting (“ASC 718”). The new standard largely aligns the accounting for share-based payment awards issued to employees and nonemployees by expanding the scope of ASC 718 to apply to nonemployee share-based transactions, as long as the transaction is not effectively a form of financing. The new guidance will be effective for the Company on January 1, 2019. The Company is currently evaluating the potential impact that this guidance may have on its consolidated financial statements. In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Requirements for Fair Value Measurement (“ASU 2018-13”). The new standard added, modified or removed disclosure requirements under Topic 820 for clarity and consistency. ASU 2018-13 is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. The Company is currently evaluating the potential impact that this guidance may have on its consolidated financial statements.

Fair value measurements

Fair value measurements	9 Months Ended
	Sep. 30, 2018
Fair value measurements
Fair value measurements	3. Fair value measurements Assets and liabilities measured at fair value on a recurring basis as of September 30, 2018 and December 31, 2017 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) As of September 30, 2018 (in thousands) Money market funds included in cash and cash equivalents $ 41,342 $ 41,342 $ — $ — Marketable securities: U.S. Treasury notes 137,459 137,459 — — Equity securities 800 800 — — Total marketable securities $ 138,259 $ 138,259 $ — $ — Warrants to purchase equity securities 356 — 356 — Total $ 179,957 $ 179,601 $ 356 $ — As of December 31, 2017 Money market funds included in cash and cash equivalents $ 30,469 $ 30,469 $ — $ — Marketable securities: U.S. Treasury notes 137,522 137,522 — — Equity securities 1,100 1,100 — — Total marketable securities $ 138,622 $ 138,622 $ — $ — Warrants to purchase equity securities 569 — 569 — Total $ 169,660 $ 169,091 $ 569 $ — The Company measures the fair value of money market funds, U.S. Treasury notes and equity securities based on quoted prices in active markets for identical securities. The Level 2 equity securities include warrants used to purchase equity securities that are valued using the Black-Scholes model. The Black-Scholes option pricing model requires inputs based on certain subjective assumptions, including (a) the expected stock price volatility, (b) the calculation of expected term of the awards, (c) the risk-free interest rate, and (d) expected dividends. The assumptions utilized to value the warrants to purchase equity securities of the Company as of September 30, 2018 and December 31, 2017 are as follows: As of September 30, As of December 31, 2018 2017 Risk-free interest rate 2.9 % 2.0 % Expected dividend yield — % — % Expected term (in years) 2.9 3.7 Expected volatility 112.0 % 103.5 % The expected volatility is based on the historic volatility for the equity securities of the Company underlying the warrants and is calculated based on a period of time commensurate with the expected term assumption. The expected term is based on the remaining contractual life of the warrants on each measurement date. The risk-free interest rate is based on a treasury instrument whose term is consistent with the expected term of the warrants. The expected dividend yield is assumed to be zero as the entity that issued the warrants has never paid and has not indicated any intention to pay dividends.

Cash, cash equivalents, and ava

Cash, cash equivalents, and available for sale marketable securities	9 Months Ended
	Sep. 30, 2018
Cash, cash equivalents, and available for sale marketable securities
Cash, cash equivalents, and available for sale marketable securities	4. Cash, cash equivalents, and available-for-sale marketable securities Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Investments consist of securities with original maturities greater than 90 days when purchased. The Company classifies these investments as available-for-sale and records them at fair value in the accompanying condensed consolidated balance sheets. Unrealized gains or losses are included in accumulated other comprehensive income (loss). Premiums or discounts from par value are amortized to investment income over the life of the underlying investment. The Company classifies marketable debt securities with a remaining maturity when purchased of greater than three months as available ‑ for ‑ sale. Marketable debt securities with a remaining maturity date greater than one year and marketable equity securities are classified as non ‑ current where the Company has the intent and ability to hold these securities for at least the next 12 months. Available ‑ for ‑ sale debt securities are maintained by an investment manager and consist of U.S. Treasury notes. In 2016, the Company invested in a supplier and received common stock and warrants to purchase common stock in that entity. The common stock is included in non-current marketable securities and the warrants are included in non-current assets. Realized gains and losses are determined using the specific identification method and are included in other income (expense). If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other ‑ than ‑ temporary” and, if so, recognizes the unrealized loss through a charge to the Company’s statement of operations and comprehensive loss. No other-than-temporary losses have been recognized. Cash, cash equivalents, and marketable securities included the following at September 30, 2018 and December 31, 2017: Amortized Unrealized Unrealized Fair Cost Gains Losses Value (in thousands) As of September 30, 2018 Money market funds included in cash and cash equivalents $ 41,342 $ — $ — $ 41,342 Marketable securities: U.S. Treasury notes 137,504 — 45 137,459 Equity securities 1,220 — 420 800 Total marketable securities $ 138,724 $ — $ 465 $ 138,259 Total money market funds and marketable securities $ 180,066 $ — $ 465 $ 179,601 As of December 31, 2017 Money market funds included in cash and cash equivalents $ 30,469 $ — $ — $ 30,469 Marketable securities: U.S. Treasury notes 137,560 — 38 137,522 Equity securities 1,220 — 120 1,100 Total marketable securities $ 138,780 $ — $ 158 $ 138,622 Total money market funds and marketable securities $ 169,249 $ — $ 158 $ 169,091 All of the Company’s marketable debt securities at September 30, 2018, have a contractual maturity of one year or less. The following table provides a reconciliation of cash, cash equivalents, and restricted cash within the condensed consolidated balance sheet that sum to the total of the same such amounts shown in the statement of cash flows: As of September 30, As of December 31, 2018 2017 (in thousands) Cash and cash equivalents $ 42,142 $ 31,530 Restricted cash included in deposits and other noncurrent assets 735 735 Total cash, cash equivalents, and restricted cash $ 42,877 $ 32,265

Accrued expenses

Accrued expenses	9 Months Ended
	Sep. 30, 2018
Accrued expenses
Accrued expenses	. Accrued expenses Accrued expenses as of September 30, 2018 and December 31, 2017 consist of the following: As of September 30, As of December 31, 2018 2017 (in thousands) Research and development costs $ 3,775 $ 5,780 Employee compensation costs 2,943 3,383 Professional services 824 1,762 Accrued goods and services 721 388 Patent costs 50 120 Other 42 64 Total $ 8,355 $ 11,497

Commitments and contingencies

Commitments and contingencies	9 Months Ended
	Sep. 30, 2018
Commitments and contingencies
Commitments and contingencies	6. Commitments and contingencies Operating Leases In April 2014, the Company entered into an agreement to lease a facility at 75 Sidney Street under a non‑cancelable operating lease that would expire December 15, 2019. The lease includes two renewal options, each for five-year terms and at fair market value upon exercise. The lease contains escalating rent clauses which require higher rent payments in future years. The Company expenses rent on a straight‑line basis over the term of the lease, including any rent‑free periods. In December 2015, the Company executed an amendment to the 75 Sidney Street lease to extend its term, and executed an agreement to lease a facility at 64 Sidney Street until December 31, 2024. The facility at 64 Sidney Street includes laboratory and office space, and was ready for occupancy in early 2017. In February 2018, the Company executed a second amendment to the 75 Sidney Street lease to lease additional space to support its continued growth. The additional facility includes laboratory and office space, and was ready for occupancy in mid-2018. In June 2018, the Company executed a third amendment to the 75 Sidney Street lease to lease additional space to further support its continued growth. The additional facility will include laboratory and office space, and will be ready for occupancy in late 2018. The third amendment extended the term of the 75 Sidney Street lease until November 30, 2026. Additionally, the Company executed a second amendment to the 64 Sidney Street lease to extend that lease until November 30, 2026. The Company received leasehold improvement incentives from the landlord of both leased properties totaling $1.6 million and $3.5 million for 75 Sidney Street and 64 Sidney Street, respectively, as of September 30, 2018. The Company recorded these incentives as a component of deferred rent and is amortizing these incentives as a reduction of rent expense over the life of the leases. The leasehold improvements have been recorded as fixed assets. The Company is entitled to receive approximately $0.3 million of leasehold improvements for the additional space at 75 Sidney Street. The following table summarizes the Company’s significant contractual obligations as of payment due date by period at September 30, 2018, related to the amended 75 Sidney Street lease and the amended 64 Sidney Street lease through December 2026: Total Minimum Lease Payments (in thousands) 2018 (remainder of year) $ 1,056 2019 4,325 2020 4,731 2021 4,863 2022 5,001 2023+ 21,924 $ 41,900 Significant Agreements Sanofi Genzyme Collaboration Agreement Summary of Agreement In February 2015, the Company entered into a collaboration agreement with Sanofi Genzyme (the “Sanofi Genzyme Collaboration Agreement”), which included a non‑refundable upfront payment of $65.0 million. In addition, contemporaneously with entering into the Sanofi Genzyme Collaboration Agreement, Sanofi Genzyme entered into a Series B Stock Purchase Agreement, under which Sanofi Genzyme purchased 10,000,000 shares of Series B Preferred Stock for $30.0 million. The fair value of the Series B Preferred Stock at the time of issuance was approximately $25.0 million. The $5.0 million premium over the fair value is accounted for as additional consideration under the Sanofi Genzyme Collaboration Agreement. Under the Sanofi Genzyme Collaboration Agreement, the Company granted Sanofi Genzyme an exclusive option to license, develop and commercialize (i) ex‑U.S. rights to the following programs, which are referred to as “Split Territory Programs”; VY‑AADC (“Parkinson’s Program”), VY‑FXN01 (“Friedreich’s Ataxia Program”), a future program to be designated by Sanofi Genzyme (“Future Program”), and VY‑HTT01 (“Huntington’s Program”) with an incremental option to co‑commercialize VY‑HTT01 in the United States and (ii) worldwide rights to VY‑SMN101 (“Spinal Muscular Atrophy Program”). Sanofi Genzyme’s option for the Split Territory Programs and the Spinal Muscular Atrophy Program is triggered following the completion of the first proof of principle human clinical study (“POP Study”), on a program-by-program basis. Prior to any option exercise by Sanofi Genzyme, the Company will collaborate with Sanofi Genzyme in the development of products under each Split Territory Program and/or the Spinal Muscular Atrophy Program pursuant to a written development plan and under the guidance of an Alliance Joint Steering Committee (“AJSC”), comprised of an equal number of employees from the Company and Sanofi Genzyme. The Company is required to use commercially reasonable efforts to develop products under each Split Territory Program and the Spinal Muscular Atrophy Program through the completion of the applicable POP Study. During the development of these joint programs, the activities are guided by a Development Advisory Committee (“DAC”). The DAC may elect to utilize certain Sanofi Genzyme technology relating to the Parkinson’s Program, the Huntington’s Program, or generally with the manufacture of Split Territory Program products. The Company is solely responsible for all costs incurred in connection with the development of the Split Territory Programs and the Spinal Muscular Atrophy Program products prior to the exercise of an option by Sanofi Genzyme with the exception of the following: (i) at the Company’s request and upon mutual agreement, Sanofi Genzyme will provide “in‑kind” services valued at up to $5.0 million and (ii) Sanofi Genzyme shall be responsible for the costs and expenses of activities under the Huntington’s Program development plan to the extent such activities are covered by financial support Sanofi Genzyme is entitled to receive from a patient advocacy group. Other than the Parkinson’s Program (for which a POP Study has already been completed), if the Company does not initiate a POP Study for a given Split Territory Program by December 31, 2026 (or for the Future Program by the tenth anniversary of the date the Future Program is nominated by Sanofi Genzyme ), and Sanofi Genzyme has not terminated the Sanofi Genzyme Collaboration Agreement with respect to the collaboration program, then Sanofi Genzyme shall be entitled, as its sole and exclusive remedy, to a credit of $10.0 million for each such program against other milestone or royalty payments payable by Sanofi Genzyme under the Sanofi Genzyme Collaboration Agreement. However, if the POP Study is not initiated due to a regulatory delay or a force majeure event, such time period shall be extended for so long as such delay continues. With the exception of the Parkinson’s Program, Sanofi Genzyme is required to pay an option exercise payment of $20.0 million or $30.0 million for each Split Territory Program and Spinal Muscular Atrophy Program. Upon Sanofi Genzyme ’s exercise of its option to license a given product in a Split Territory Program (“Split Territory Licensed Product”), the Company will have sole responsibility for the development of such Split Territory Licensed Product in the United States and Sanofi Genzyme shall have sole responsibility for development of such Split Territory Licensed Product in the rest of the world. The Company and Sanofi Genzyme will have shared responsibility for execution of ongoing development of such Split Territory Licensed Product that is not specific to either territory, including costs associated therewith. The Company is responsible for all commercialization activities relating to Split Territory Licensed Products in the United States, including all of the associated costs. Sanofi Genzyme is responsible for all commercialization activities relating to the Split Territory Licensed Products in the rest of the world, including all of the associated costs. If Sanofi Genzyme exercises its co‑commercialization rights, Sanofi Genzyme will be the lead party responsible for all commercialization activities related to the Huntington’s Program product (the “Huntington’s Licensed Product”) in the United States. Upon exercise of Sanofi Genzyme shall have the sole right to develop the Spinal Muscular Atrophy Sanofi Genzyme shall be responsible for all of the development costs that occur after the option exercise date for the Spinal Muscular Atrophy Program. Sanofi Genzyme is also responsible for worldwide commercialization activities relating to the Spinal Muscular Atrophy In October 2017, Sanofi Genzyme decided not to exercise its option for the Parkinson’s Program (the “PD Opt-Out”). Sanofi Genzyme is required to pay the Company for specified regulatory and commercial milestones, if achieved, up to $540.0 million across all remaining programs. The Company is no longer entitled to receive a total of $105.0 million related to regulatory and commercial milestone payments for the Parkinson’s Program as a result of the PD Opt-Out. The regulatory approval milestones are payable upon either regulatory approval in the United States or regulatory and reimbursement approval in the European Union and range from $40.0 million to $50.0 million per milestone, with an aggregate total of $220.0 million, after accounting for the PD Opt-Out. The commercial milestones are payable upon achievement of specified annual net sales in each program and range from $50.0 million to $100.0 million per milestone, with an aggregate total of $320.0 million, after accounting for the PD Opt-Out . In addition, to the extent any Split Territory Licensed Products or the Spinal Muscular Atrophy Licensed Product are commercialized, the Company is entitled to tiered royalty payments ranging from the mid‑single digits to mid‑teens based on a percentage of net sales by Sanofi Genzyme . Sanofi Genzyme is entitled to receive tiered royalty payments related to sales of a Split Territory Licensed Product ranging from the low‑single digits to mid‑single digits based on a percentage of net sales by the Company depending on whether the Company uses Sanofi Genzyme technology in the Split Territory Licensed Product. If Sanofi Genzyme elects to co‑commercialize the Huntington’s Licensed Product in the United States, the Company and Sanofi Genzyme will share in any profits or losses from Huntington’s Licensed Product sales. The Sanofi Genzyme Collaboration Agreement will continue in effect until the later of (i) the expiration of the last to expire of the option rights and (ii) the expiration of all payment obligations unless sooner terminated by the Company or Sanofi Genzyme . The Company and Sanofi Genzyme have customary termination rights, including the right to terminate for an uncured material breach of the agreement committed by the other party and Sanofi Genzyme has the right to terminate for convenience. Accounting Analysis At inception, the Sanofi Genzyme Collaboration Agreement included the following performance obligations: (i) research and development services for each of the Split Territory Programs and the Spinal Muscular Atrophy Program, (ii) participation in the AJSC, (iii) participation in the DAC and (iv) a material right associated with an option to obtain a development and commercial license in the Parkinson’s Program (“PD Material Right”). The Company determined that the option to obtain a development and commercial license in the Parkinson’s Program was a material right under ASC 606 primarily because there were no additional option exercise payments payable by Sanofi Genzyme at the time of option exercise. Therefore, the PD Material Right was considered a performance obligation at the inception of the arrangement. The options in the other Split Territory Programs and the Spinal Muscular Atrophy Program do not provide a material right to the customer that it would receive without entering into the contract principally because the option fees are at least equal to the standalone selling price for the underlying goods. Therefore, the other Split Territory Programs and the Spinal Muscular Atrophy Program options are not performance obligations at inception. The Company has identified $74.6 million of total transaction price consisting of the $65.0 million upfront fee, the $5.0 million premium paid in excess of fair value of the Series B Preferred Stock and $4.6 million of Sanofi Genzyme “in‑kind” funding, which represents the transaction price at adoption. Additional consideration to be paid to the Company upon the exercise of the license options by Sanofi Genzyme or upon reaching certain milestones are excluded from the transaction price as they relate to option fees and milestones that can only be achieved subsequent to the option exercise or are outside of the initial contract term. The Company has allocated the transaction price to the separate performance obligations based on their relative standalone selling price. For all performance obligations, the Company determined the standalone selling price at contract inception based on each obligation’s estimated standalone selling price (“ESP”). The Company determined the ESP for the service related deliverable for the research and development activities based on internal estimates of the costs to perform the services, including expected internal expenses and expenses with third parties for services and supplies, marked up to include a reasonable profit margin and adjusted for the scope of the potential license. Significant inputs used to determine the total expense of the research and development activities include, the length of time required and the number and costs of various studies that will be performed to complete the applicable POP Study. The ESP 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 ESP for the PD Material Right was determined based on the estimated value of the license adjusted for the estimated probability that the option would be exercised by Sanofi Genzyme. Based on the relative standalone selling price allocation, the allocation of the transaction price to the separate performance obligations was as follows: Performance Obligation Amount (in thousands) Research and Development Services for: Huntington’s Program $ 14,228 Parkinson’s Program 6,040 Friedreich’s Ataxia Program 14,821 Spinal Muscular Atrophy Program 29,116 Future Program 2,239 Committee Obligations: AJSC 133 DAC 207 PD Material Right 7,855 Total $ 74,639 The Company recognizes the amounts associated with research and development services and committee obligations on a proportional performance basis over the period of service using input-based measurements such as costs incurred to date, to estimate proportion performed, and remeasures its progress towards completion at the end of each reporting period. The amount allocated to the PD Material Right was initially deferred and recognized in full prior to the adoption of ASC 606. During the three and nine months ended September 30, 2018, the Company recognized $0.1 million and $1.3 million, respectively, of revenue associated with its collaboration with Sanofi Genzyme related to research and development services and committee obligations performed during the period. As of September 30, 2018, there was $50.4 million of deferred revenue related to the Sanofi Genzyme Collaboration Agreement, which is classified as either current or noncurrent in the accompanying balance sheet based on the period the services are expected to be delivered. Contingent consideration related to the performance of in-kind services is considered in the transaction price based on the most-likely amount, which is the full amount of the services that the Company can require Sanofi Genzyme to complete. There is no other available contingent consideration until such point as Sanofi Genzyme exercises an option to one of the research programs. Costs incurred relating to the programs that Sanofi Genzyme has the option to license under the Sanofi Genzyme Collaboration Agreement consist of internal and external research and development costs, which primarily include: salaries and benefits, lab supplies and preclinical research studies. These costs are included in research and development expenses in the Company’s statement of operations during the three and nine months ended September 30, 2018 and 2017. AbbVie Collaboration Agreement Summary of Agreement In February 2018, the Company entered into an exclusive collaboration and option agreement (the “AbbVie Collaboration Agreement”) with AbbVie Biotechnology Ltd (“AbbVie”) for the research, development and commercialization of AAV and other virus-based gene therapy products for the treatment of diseases of the central nervous system and other neurodegenerative diseases related to defective or excess aggregation of tau protein in the human brain, including Alzheimer’s disease. Under the AbbVie Collaboration Agreement, the Company and AbbVie have agreed to collaborate on the research and development of specified vectorized antibody compounds comprised of an AAV or other viral capsid and a virus vector genome that encodes one or more antibodies that target and bind to a tau protein. The collaboration is comprised of a research period (the “Research Period”), a development period (the “Development Period”), and an exclusive license option (the “License Option”). The AbbVie Collaboration Agreement included a non-refundable upfront payment of $69.0 million for services during the Research Period. During the Research Period, each party has agreed to identify up to five antibodies for inclusion in the collaboration. Subject to certain conditions and exceptions, the parties will select up to three antibodies (each, a “Research Antibody”) as candidates for creation of research compounds (each, a “Research Compound”), with AbbVie having the right to select two of the three Research Antibodies. The Company is required to use diligent efforts to conduct antibody engineering and other research activities to create Research Compounds and to develop product candidates containing or comprised of such Research Compounds (“Product Candidates”). The Company is solely responsible for its costs and expenses during the Research Period. During a specified portion of the Research Period, AbbVie may exercise one or more of its exclusive development options (each, a “Development Option”) to select up to a total of three Research Compounds (the “Selected Research Compounds”) and their corresponding Product Candidates (the “Selected Product Candidates”) to proceed to the Development Period. Upon AbbVie’s exercise of a Development Option, AbbVie will pay the Company $80.0 million for the first Selected Research Compound and $30.0 million each for up to two additional Selected Research Compounds. During the Development Period, the Company is obligated to use diligent efforts to conduct development activities, including Investigational New Drug application-enabling and Phase 1 clinical trial activities, for the Selected Research Compounds and corresponding Selected Product Candidates. The Company will be solely responsible for the costs and expenses during the Development Period. During a specified portion of the Development Period (the “License Option Period”), AbbVie may exercise its License Option to further develop and commercialize all of the Research Compounds (the “Licensed Compounds”), and corresponding product candidates (the “Licensed Products”). Upon AbbVie’s exercise of its License Option, AbbVie will provide a one-time payment of $75.0 million to the Company, and the Company will grant to AbbVie an exclusive, worldwide license, with the right to sublicense, under certain of the Company’s intellectual property rights to develop and commercialize the Licensed Compounds and the Licensed Products for all human diagnostic, prophylactic and therapeutic uses. In addition, after AbbVie’s exercise of the License Option, the Company has certain obligations to complete any remaining research and development activities that have not been completed for any Research Compounds and Product Candidates. The Company’s research and development activities will be conducted pursuant to the plans agreed to by the parties and overseen by a joint governance committee (“JGC”) as detailed in the AbbVie Collaboration Agreement. Any material amendment to the research or development plans must be mutually agreed to by the Company and AbbVie, which may be through the JGC. Under the AbbVie Collaboration Agreement, AbbVie is required to use commercially reasonable efforts to develop and commercialize at least one Licensed Product in each of the United States, Japan, the United Kingdom, Germany, France, Italy, and Spain. After exercise of the License Option, AbbVie is solely responsible for all development and commercialization activities relating to Licensed Compounds and Licensed Products at its sole cost and expense, subject to the agreed-upon research and development plans. The Company may elect to share in AbbVie’s development costs relating to a Licensed Product on an indication-by-indication basis in exchange for a specified increase in royalties (a “Cost-Sharing Option”). If the Company exercises a Cost-Sharing Option, the Company may either reimburse AbbVie for AbbVie’s applicable development costs or, in the case of certain budget overruns, AbbVie may instead deduct applicable development costs, up to a specified cap, from milestone and royalty payments owed by AbbVie to the Company. Under the AbbVie Collaboration, the Company is eligible to receive specified development and first-sale milestone payments for each Licensed Compound of up to an aggregate of $550.0 million in the case of an Alzheimer’s disease indication, up to $230.0 million in the case of the first indication other than Alzheimer’s disease and up to $115.0 million for a subsequent non-Alzheimer’s disease indication. Additionally, the Company is eligible to receive tiered, escalating royalties, in a range from a high-single digit to a mid-to-high teen (or, if the Company has exercised its Cost-Sharing Option, low-twenties) percentage of aggregate net sales of Licensed Products on a Licensed Compound by Licensed Compound basis, subject to potential reductions in certain circumstances. For each Licensed Product, AbbVie also has the right to decrease or eliminate its royalty payments on such Licensed Product in exchange for a one-time payment by AbbVie at a fair market value to be negotiated by the parties or determined pursuant to dispute resolution procedures specified in the AbbVie Collaboration Agreement. Unless earlier terminated, the AbbVie Collaboration Agreement will expire on the earliest to occur of the expiration of (i) the Development Option Period, without AbbVie’s exercise of a Development Option; (ii) the License Option Period, without AbbVie’s exercise of its License Option; and (iii) the last-to-expire royalty term with respect to all Licensed Products in all countries. The Company and AbbVie have customary termination rights including the right to terminate for an uncured material breach of the agreement committed by the other party, and AbbVie has the right to terminate for convenience. Accounting Analysis The Company assessed the promised goods and services under the AbbVie Collaboration Agreement, in accordance with ASC 606, and determined that the AbbVie Collaboration Agreement includes the following performance obligations: (i) research services during the Research Period (through the delivery of the final research report) including the identification of the Research Antibodies, conduct of research activities and provision of information to AbbVie to allow AbbVie to determine whether to exercise up to three Development Options to be rendered (collectively, the “Research Services”), and (ii) a material right associated with the Development Option on the first Research Compound and associated Product Candidates (“First Development Option Material Right”). The first Development Option provides AbbVie with (i) additional development services on a selected Research Compound and (ii) the ability to exercise the License Option. The Company has concluded the option provides a material right as the consideration paid by AbbVie upon exercise of the first Development Option is less than the amount that the Company would otherwise expect to receive outside the context of the contract. The Company has concluded that the First Development Option Material Right is a separate performance obligation under ASC 606 as AbbVie is provided additional services and a License Option for additional consideration that represents a significant discount from amounts that would otherwise be offered outside the context of the contract. The First Development Option Material Right is distinct from the other performance obligations in the arrangement as it is an option in the contract that is not required for AbbVie to obtain the benefit of the other promised goods or services in the arrangement. The First Development Option Material Right does not include the underlying goods or services that are delivered upon exercise of the option, but rather represents the value to the customer of having the right to obtain development services and the right to the License Option at an advantageous price. The Company received a nonrefundable, upfront payment of $69.0 million as consideration under the AbbVie Collaboration Agreement, which represents the transaction price at inception. Additional consideration to be paid to the Company upon the exercise of the Development and License Options by AbbVie or upon reaching certain milestones are excluded from the transaction price as they relate to option fees and milestones that can only be achieved subsequent to the option exercise or are outside of the initial contact term. The Company has allocated the transaction price to the separate performance obligations based on their relative standalone selling price. The Company determined the standalone selling price at contract inception based on each obligation’s ESP. The Company determined the ESP for the research services obligation based on internal estimates of the costs to perform the services, including expected internal expenses and expenses with third parties for services and supplies, inclusive of a reasonable profit margin. Significant inputs used to determine the total expense of the research services include the length of time required, the internal hours expected to be incurred on the services and the number and costs of various studies that will be performed to complete the Research Plan. The ESP for the First Development Option Material Right was determined based on the fees AbbVie would pay to exercise the Development and License Options, the estimated costs to perform the development services, inclusive of a reasonable profit margin, the estimated value of the License Option using comparable transactions, and the probability that the Development and License Options would be exercised by AbbVie. Based on the relative standalone selling price, the allocation of the transaction price to the separate performance obligations was as follows: Performance Obligation Amount (in thousands) Research Services $ 34,482 First Development Option Material Right 34,518 Total $ 69,000 The Company recognizes the amounts associated with Research Services on a proportional performance basis over the period of service using input-based measurements of total cost of research incurred to estimate proportion performed and remeasures its progress towards completion at the end of each reporting period. The amount allocated to the First Development Option Material Right is recorded as deferred revenue and will be recognized either over the period in which goods and services underlying the option are transferred or upon expiry of the option. During the three and nine months ended September 30, 2018, the Company recognized $2.0 million and $4.3 million, respectively, of revenue associated with the AbbVie Collaboration related to the Research Services performed during the period. As of September 30, 2018, there is $64.7 million of deferred revenue related to the AbbVie Collaboration Agreement, which is classified as either current or noncurrent in the accompanying balance sheet based on the period the goods or services are expected to be delivered. Costs incurred relating to the AbbVie Collaboration Agreement consist of internal and external research and development costs, which primarily include: salaries and benefits, lab supplies, and preclinical research studies. All of these costs are included in research and development expenses in the Company’s statement of operations during the three and nine months ended September 30, 2018. MRI Interventions License and Securities Purchase Agreements In September 2016, the Company entered into ® System, which is being used by the Company in ongoing development and clinical trials. Under In May 2018, the Company entered into a master services and supply agreement with MRIC (the “MRIC Supply Agreement”) which provides for MRIC to perform certain manufacturing, supply, development and services as requested by the Company, including the supply of the ClearPoint System and cannula devices. In the three and nine months ended September 30, 2018, the Company has incurred $0.1 million and $0.2 million of expense, respectively, related to the MRIC Supply Agreement. Other Agreements During September 2016, the Company entered into a research and development funding arrangement with a non-profit organization that provides up to $4.0 million in funding upon the achievement of clinical and development milestones. The agreement provides that the Company repay amounts received under certain circumstances including termination of the agreement, and to pay an amount up to 2.6 times the funding received upon successful development and commercialization of any products developed. During 2017, the Company earned a milestone payment of $1.0 million. During the three and nine months ended September 30, 2018, the Company did not earn any additional milestone payments. The Company has evaluated the arrangement and has concluded that it represents a research and development financing arrangement as it is probable that the Company will repay amounts received under the arrangement. As a result, the $1.0 million earned to date is recorded as a non-current liability in the accompanying balance sheet. Litigation The Company is not a party to any material legal matters or claims and did not have contingency reserves established for any litigation liabilities as of September 30, 2018 or December 31, 2017.

Redeemable convertible preferre

Redeemable convertible preferred stock	9 Months Ended
	Sep. 30, 2018
Redeemable convertible preferred stock
Redeemable convertible preferred stock	7. Redeemable convertible preferred stock The Company has authorized preferred stock amounting to 5,000,000 shares as of September 30, 2018 and December 31, 2017. The authorized preferred stock was classified under stockholders’ equity at September 30, 2018 and December 31, 2017.

Stock based compensation

Stock based compensation	9 Months Ended
	Sep. 30, 2018
Stock based compensation
Stock based compensation	8. Stock‑based compensation 2014 Stock Option and Grant Plan In January 2014, the Company adopted the 2014 Stock Option and Grant Plan (the “2014 Plan”), under which it could grant incentive stock options, non‑qualified stock options, restricted stock awards, unrestricted stock awards, or restricted stock units to purchase up to 823,529 shares of common stock to employees, officers, directors and consultants of the Company. In April 2014, the Company amended the 2014 Plan to allow for the issuance of up to 1,411,764 shares of common stock. In August 2014, April 2015, August 2015, and October 2015, the Company further amended the 2014 Plan to allow for the issuance of up to 2,000,000, 2,047,058, 2,669,411, and 2,998,823 shares of common stock, respectively. During 2014 the Company issued only restricted stock awards under the 2014 Plan and during 2015 the Company only granted stock options under the 2014 Plan. The terms of equity award agreements, including vesting requirements, are determined by the Board of Directors and are subject to the provisions of the 2014 Plan. Founder Awards In January 2014, the Company issued 1,188,233 shares of restricted stock to its founders at an original issuance price of $0.0425 per share. Of the total restricted shares awarded to the founders, 835,292 shares generally vested 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 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 is recognized when the achievement of the performance condition is considered probable, using management’s best estimates. During 2016, management determined that the achievement of the performance milestone for one of the three performance‑based awards had become probable and began recognizing stock-based compensation accordingly. The Company recorded $0.1 million and $0.3 million in stock-based compensation expense related to this award during the three and nine months ended September 30, 2018, respectively. The Company recorded $0.3 million and $0.9 million in stock-based compensation expense related to this award during the three and nine months ended September 30, 2017, respectively. No stock‑based compensation expense was recorded for the remaining two founders’ awards with performance-based vesting as of September 30, 2018 as the performance-based milestones related to these awards were not probable. 2015 Stock Option Plan In October 2015, the Company’s board of directors and stockholders approved the 2015 Stock Option and Incentive Plan (the “2015 Stock Option Plan”) which became effective upon the completion of the IPO. The 2015 Stock Option Plan provides the Company with the flexibility to use various equity-based incentive and other awards as compensation tools to motivate its workforce. These tools include stock options, stock appreciation rights, restricted stock, restricted stock units, unrestricted stock, performance share awards and cash-based awards. The 2015 Stock Option Plan replaced the 2014 Plan. Any options or awards outstanding under the 2014 Plan remained outstanding and effective. The number of shares initially reserved for issuance under the 2015 Stock Option Plan is the sum of (i) 1,311,812 shares of common stock and (ii) the number of shares under the 2014 Plan that are not needed to fulfill the Company’s obligations for awards issued under the 2014 Plan as a result of forfeiture, expiration, cancellation, termination or net issuances of awards thereunder. The number of shares of common stock that may be issued under the 2015 Stock Option Plan is also subject to increase on the first day of each fiscal year by up to 4% of the Company’s issued and outstanding shares of common stock on the immediately preceding December 31. Effective January 1, 2016, 2017, and 2018, an additional 1,069,971, 1,070,635, and 1,285,200 shares, respectively, were added to the Company’s 2015 Stock Option Plan for future issuance. As of September 30, 2018, there were 2,055,980 shares of common stock available for future award grants under the 2015 Stock Option Plan. During the three and nine months ended September 30, 2018, the Company issued a total of 846,000 and 2,030,191 stock options, respectively, to employees and directors under the 2015 Stock Option Plan. During the three and nine months ended September 30, 2018, there were no new stock options issued to non-employees under the 2015 Stock Option Plan. 2015 Employee Stock Purchase Plan In October 2015, the Company’s board of directors and stockholders approved the 2015 Employee Stock Purchase Plan (the “2015 ESPP”). The 2015 ESPP became effective upon the closing of the IPO. Under the 2015 ESPP, all full-time employees of the Company are eligible to purchase common stock of the Company twice per year, at the end of each six-month payment period. During each payment period, eligible employees who so elect, may authorize payroll deductions in an amount of 1% to 10% (whole percentages only) of the employee’s base pay for each payroll period. At the end of each payment period, the accumulated deductions are used to purchase shares of common stock from the Company at a discount. A total of 262,362 shares of common stock were initially authorized for issuance under this plan. The number of shares of common stock that may be issued under the 2015 ESPP is also subject to increase on the first day of each fiscal year by up to 1% of the Company’s issued and outstanding shares of common stock on the immediately preceding December 31. Effective January 1, 2016, 2017, and 2018, 267,492, 267,658, and 321,300 shares of common stock, respectively, were added to the 2015 ESPP. Inducement Stock Option Awards On July 16, 2018, in connection with his commencement of employment as the president and chief executive officer of the Company, the Company issued to G. Andre Turenne a non-statutory stock option to purchase an aggregate of 650,000 shares of the Company’s common stock at an exercise price of $18.03 per share. Subject to Mr. Turenne’s continued service to the Company, the stock option will vest over a four-year period, with 25% of the shares underlying the option award vesting on the first anniversary of the award and the remaining 75% of the shares underlying the award vesting monthly thereafter over the subsequent 36-month period. The stock option was issued outside of the Company’s 2015 Stock Option Plan as an inducement material to Mr. Turenne’s acceptance of an offer of employment with the Company in accordance with Nasdaq Listing Rule 5635(c)(4). Stock‑Based Compensation Expense Total compensation cost recognized for all stock‑based compensation awards in the statements of operations and comprehensive loss is as follows: Three Months Ended Nine Months Ended September 30, September 30, 2018 2017 2018 2017 (in thousands) Research and development $ 1,176 $ 2,231 $ 3,706 $ 4,107 General and administrative 1,460 1,010 9,366 2,838 Total stock compensation expense $ 2,636 $ 3,241 $ 13,072 $ 6,945 Restricted Stock A summary of the status of and changes in unvested restricted stock activity under the Company’s equity award plans for the nine months ended September 30, 2018 was as follows: Weighted Average Grant Date Fair Value Shares Per Share Unvested restricted common stock as of December 31, 2017 557,979 $ 0.70 Issued — — Vested (318,211) $ 0.84 Repurchased (2,794) $ 1.11 Unvested restricted common stock as of September 30, 2018 236,974 $ 0.51 The expense related to awards granted to employees and non-employees was $4.6 thousand and $18.5 thousand, respectively, for the three months ended September 30, 2018, and $0.1 million and $0.4 million for the nine months ended September 30, 2018, respectively. The expense related to awards granted to employees and non-employees was $0.1 million and $1.6 million, respectively, for the three months ended September 30, 2017, and $0.4 million and $2.3 million for the nine months ended September 30, 2017, respectively. As of September 30, 2018, the Company had unrecognized stock-based compensation expense related to its unvested restricted stock awards of $0.1 million, which is expected to be recognized over the remaining average vesting period of 0.2 years. Stock Options The following is a summary of stock option activity for the nine months ended September 30, 2018: Weighted Remaining Aggregate Average Contractual Intrinsic Exercise Life Value Shares Price (in years) (in thousands) Outstanding at December 31, 2017 3,143,566 $ 11.82 Granted 2,030,191 $ 21.54 Exercised (360,099) $ 10.17 Cancelled or forfeited (647,172) $ 16.20 Outstanding at September 30, 2018 4,166,486 $ 16.02 8.6 $ 17,613 Exercisable at September 30, 2018 1,375,760 $ 12.02 7.5 $ 10,288 Vested and expected to vest at September 30, 2018 4,166,486 $ 16.02 8.6 $ 17,613 Using the Black ‑ Scholes option pricing model, the weighted-average fair value of options granted to employees and directors during the three and nine months ended September 30, 2018 was $12.20 and $14.37 per share, respectively. The expense related to options granted to employees and directors was $2.5 million and $12.3 million for the three and nine months ended September 30, 2018, respectively. The weighted-average fair value of options granted to employees and directors during the three and nine months ended September 30, 2017 was $9.43 and $7.88 per share, respectively. The expense related to awards granted to employees and directors was $1.4 million and $3.8 million for the three and nine months ended September 30, 2017, respectively. The fair value of each option issued to employees and directors was estimated at the date of grant using the Black‑Scholes option pricing model with the following weighted‑average assumptions: Three Months Ended Nine Months Ended September 30, September 30, 2018 2017 2018 2017 Risk-free interest rate 2.8 % 1.8 % 2.7 % 2.0 % Expected dividend yield — % — % — % — % Expected term (in years) 6.0 6.0 6.0 6.0 Expected volatility 74.5 % 72.9 % 74.4 % 73.8 % There were no new options granted to non-employees during the three and nine months ended September 30, 2018. The Company granted options to purchase 20,000 shares of its common stock to non-employees during the three and nine months ended September 30, 2017. Unvested options granted to non ‑ employees are revalued at each measurement period until fully vested. The expense related to stock option awards granted to non ‑ employees was $0.1 million and $0.3 million for the three and nine months ended September 30, 2018, respectively. The expense related to awards granted to non-employees was $0.1 million and $0.3 million for the three and nine months ended September 30, 2017, respectively. The fair value of each option issued to non‑employees was estimated at each vesting and reporting date using the Black‑Scholes option pricing model. The reporting date fair value was determined using the following weighted‑average assumptions: As of September 30, 2018 2017 Risk-free interest rate 3.0 % 2.2 % Expected dividend yield — % — % Expected term (in years) 7.9 8.7 Expected volatility 73.9 % 77.5 % As of September 30, 2018, the Company had unrecognized stock-based compensation expense related to its unvested stock options of $27.7 million which is expected to be recognized over the remaining weighted-average vesting period of 2.9 years. For the three and nine months ended September 30, 2018 and 2017, d ue to a lack of company-specific historical and implied volatility data, the Company has based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. In the three and nine months ended September 30, 2018, the Company’s estimate has also included the volatility of its common stock for the most recent period .

Income taxes

Income taxes	9 Months Ended
	Sep. 30, 2018
Income taxes
Income taxes	9. Income taxes Deferred tax assets and deferred tax liabilities are recognized based on temporary differences between the financial reporting and tax basis of assets and liabilities using statutory rates. A valuation allowance is recorded against deferred tax assets if it is more likely than not that some or all of the deferred tax assets will not be realized. Due to the uncertainty surrounding the realization of the favorable tax attributes in future tax returns, the Company has recorded a full valuation allowance against the Company’s otherwise recognizable net deferred tax assets. For the three and nine months ended September 30, 2018, the Company recognized a de minimis tax expense in other comprehensive income related to the unrealized gain on available-for-sale securities. For the three and nine months ended September 30, 2017, the Company recognized a tax expense in other comprehensive income of $0.1 million related to the unrealized gain on available-for-sale securities.

Net loss per share

Net loss per share	9 Months Ended
	Sep. 30, 2018
Net loss per share
Net loss per share	10. Net loss per share The following table sets forth the outstanding potentially dilutive securities that have been excluded in the calculation of diluted net loss per share because to include them would be anti-dilutive: As of September 30, 2018 2017 Unvested restricted common stock 236,974 676,148 Shares reserved for issuance under stock plans 7,212,380 3,044,614 Total 7,449,354 3,720,762

Related party transactions

Related party transactions	9 Months Ended
	Sep. 30, 2018
Related party transactions
Related party transactions	11. Related‑party transactions During the three and nine months ended September 30, 2018 and 2017, the Company received consulting and management services from one of its investors. The total amount of consulting and management services provided by this investor was de minimis during the three and nine months ended September 30, 2018 and 2017.

Summary of significant accoun_2

Summary of significant accounting policies (Policies)	9 Months Ended
	Sep. 30, 2018
Summary of significant accounting policies
Basis of presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements of the Company have been prepared in accordance with accounting principles generally accepted in the United States (“GAAP”) for interim financial reporting and as required by Regulation S-X, Rule 10-01. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. For further information, refer to the consolidated financial statements and footnotes included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017 as filed with the Securities and Exchange Commission (“SEC”). These interim condensed consolidated financial statements, in the opinion of management, reflect all normal recurring adjustments necessary for a fair presentation of the Company’s financial position and results of operations for the periods presented. Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”).
Principles of consolidation	Principles of Consolidation The unaudited interim consolidated financial statements include the accounts of the Company and its wholly owned subsidiary as disclosed in Note 2, Summary of Significant Accounting Policies, within the “Notes to Consolidated Financial Statements” accompanying its Annual Report on Form 10-K for the fiscal year ended December 31, 2017. Intercompany accounts and transactions have been eliminated.
Use of estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. On an ongoing basis, the Company’s management evaluates its estimates, which include, but are not limited to, estimates related to revenue recognition, accrued expenses, 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.
Revenue Recognition	Revenue Recognition The Company enters into collaboration agreements which are within the scope of ASC 606, Revenue from Contracts with Customers (“ASC 606”), under which the Company licenses rights to certain of the Company’s product candidates and performs research and development services. The terms of these arrangements typically include payment of one or more of the following: non-refundable, upfront fees; reimbursement of research and development costs; development, regulatory, and commercial milestone payments; and royalties on net sales of licensed products. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine the appropriate amount of revenue to be recognized for arrangements determined to be within the scope of ASC 606, the Company performs the following five steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect consideration it is entitled to in exchange for the goods or services it transfers to the customer. The promised goods or services in the Company’s arrangement typically consist of license rights to the Company’s intellectual property and research and development services. The Company provides options to additional items in the contracts, which are accounted for as separate contracts when the customer elects to exercise such options, unless the option provides a material right to the customer. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the outset of the arrangement. Performance obligations are promised goods or services in a contract to transfer a distinct good or service to the customer and are considered distinct when (i) the customer can benefit from the good or service on its own or together with other readily available resources and (ii) the promised good or service is separately identifiable from other promises in the contract. In assessing whether promised goods or services are distinct, the Company considers factors such as the stage of development of the underlying intellectual property, the capabilities of the customer to develop the intellectual property on its own or whether the required expertise is readily available and whether the goods or services are integral or dependent to other goods or services in the contract. The Company estimates the transaction price based on the amount expected to be received for transferring the promised goods or services in the contract. The consideration may include fixed consideration or variable consideration. At the inception of each arrangement that includes variable consideration, the Company evaluates the amount of potential payments and the likelihood that the payments will be received. The Company utilizes either the most likely amount method or expected amount method to estimate the amount expected to be received based on which method best predicts the amount expected to be received. The amount of variable consideration which is included in the transaction price may be constrained, and is included in the transaction price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized will not occur in a future period. The Company’s contracts often include development and regulatory milestone payments which are assessed under the most likely amount method and constrained if it is probable that a significant revenue reversal would occur. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At the end of each reporting period, the Company re-evaluates the probability of achievement of such development milestones and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect collaboration revenues in the period of adjustment. To date, the Company has not recognized any consideration related to the achievement of development, regulatory, or commercial milestone revenue resulting from any of the Company’s collaboration arrangements. For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any consideration related to sales-based royalty revenue resulting from any of the Company’s collaboration arrangements. The Company allocates the transaction price based on the estimated stand-alone selling price of each of the performance obligations. The Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company utilizes key assumptions to determine the stand-alone selling price for service obligations, which may include other comparable transactions, pricing considered in negotiating the transaction and the estimated costs. Additionally, in determining the standalone selling price for material rights, the Company utilizes comparable transactions, clinical trial success probabilities, and estimates of option exercise likelihood. Variable consideration is allocated specifically to one or more performance obligations in a contract when the terms of the variable consideration relate to the satisfaction of the performance obligation and the resulting amounts allocated are consistent with the amounts the Company would expect to receive for the satisfaction of each performance obligation. The consideration allocated to each performance obligation is recognized as revenue when control is transferred for the related goods or services. For performance obligations which consist of licenses and other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Upfront payments and fees are recorded as deferred revenue upon receipt or when due until the Company performs its obligations under these arrangements. Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional.
Recent accounting pronouncements	Recently Adopted Accounting Pronouncements In January 2016, the FASB issued ASC Update No. 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities (“Update No. 2016-01”). The purpose of Update No. 2016-01 is to improve financial reporting for financial instruments by reducing the number of items recorded to other comprehensive income. The Company adopted Update No. 2016-01 in the first quarter of 2018, using the modified retrospective method. Unrealized gains and losses previously recorded to other comprehensive income (loss) were reclassified to accumulated deficit and all future fair value changes will be recorded to other income (loss). The adoption of the standard on January 1, 2018 did not have a material impact on the Company’s consolidated financial statements. In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (“ASU 2016-15”), which simplifies certain elements of cash flow classification and is intended to reduce diversity in practice in how certain transactions are classified in the statement of cash flows. The update is effective for fiscal years, and interim periods within those years, beginning after December 15, 2017. The Company adopted ASU 2016-15 on January 1, 2018, and such adoption did not have a material impact on the Company's consolidated financial statements. In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash (“ASU 2016-18”). The amendments in ASU 2016-18 require an entity to reconcile and explain the period-over-period change in total cash, cash equivalents and restricted cash within its statements of cash flows and was applied utilizing a full retrospective approach. The Company adopted the new standard on January 1, 2018. The Company has included the necessary reconciliation within Note 4 “Cash, cash equivalents, and available-for-sale marketable securities.” In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (“ASU 2014-09”), which supersedes the revenue recognition requirements in ASC 605, Revenue Recognition (“ASC 605”), and most industry-specific guidance. The new standard requires that an entity recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The update also requires additional disclosure about the nature, amount, timing, and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. Thereafter, a series of clarifying ASUs, narrow scope improvements and practical expedients were issued. This collective guidance resulted in the new revenue standard, ASC 606. The Company adopted the new standard effective January 1, 2018 using the modified retrospective approach. The Company had one open contract, relating to the Sanofi Genzyme Collaboration, on the adoption date and has assessed it under the new revenue standard. The adoption of ASC 606 resulted in the changes to (i) the allocation of arrangement consideration, including the determination of estimated selling price and the allocation of variable consideration to specific performance obligations and (ii) the application of proportional performance as a measure of progress on service related deliverables. The Company has accounted for the impact of adopting ASC 606 as a cumulative catch-up under the modified retrospective approach, which is represented as an increase of $20.0 million to deferred revenue with an offset to accumulated deficit, effective January 1, 2018. The following financial statement line items have been shown to reflect comparative balances under ASC 606 and ASC 605 for each of the three-month period and nine-month period ended September 30, 2018, for both of the Sanofi Genzyme Collaboration and AbbVie Collaboration, collectively. Condensed Consolidated Statements of Operations and Comprehensive Loss Three months ended September 30, 2018 Under ASC 606 Under ASC 605 Effect of change (in thousands, except per share data) Collaboration revenue $ 2,094 $ 3,054 $ (960) Loss before income taxes (20,289) (19,329) (960) Net loss (20,289) (19,329) (960) Net loss per share, basic and diluted (0.63) (0.60) (0.03) Nine months ended September 30, 2018 Under ASC 606 Under ASC 605 Effect of change (in thousands, except per share data) Collaboration revenue $ 5,611 $ 7,267 $ (1,656) Loss before income taxes (65,937) (64,281) (1,656) Net loss (65,757) (64,101) (1,656) Net loss per share, basic and diluted (2.06) (2.00) (0.06) Condensed Consolidated Balance Sheets As of September 30, 2018 Under ASC 606 Under ASC 605 Effect of change (in thousands) Deferred revenue, current $ 20,250 $ 19,078 $ 1,172 Deferred revenue, non-current 94,804 74,390 20,414 Accumulated deficit (246,520) (224,934) (21,586) Condensed Consolidated Statements of Cash Flows Nine months ended September 30, 2018 Under ASC 606 Under ASC 605 Effect of change (in thousands) Net loss $ (65,757) $ (64,101) $ (1,656) Adjustments to reconcile net loss to net cash provided by operating activities: Deferred revenue 63,388 61,732 1,656 Recent Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, a comprehensive new lease accounting standard, which provides revised guidance on accounting for lease arrangements by both lessors and lessees and requires lessees to recognize a lease liability and a right-of-use asset for most leases. In July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842), Targeted Improvements , which provides an additional transition method that allows entities to initially apply the new lease requirements at the adoption date, not the earliest period presented, and recognize a cumulative effect adjustment to the opening balance of retained earnings in the period of adoption. The Company expects to elect this transition method at the adoption date of January 1, 2019. The Company also expects to elect a package of practical expedients, under which an entity need not reassess whether any expired or existing contracts are or contain leases, the lease classification for any expired or existing leases, or initial direct costs for any existing leases. The Company is currently in the process of evaluating the impact that the adoption of this guidance will have on its consolidated financial statements and related disclosures. On December 22, 2017, the SEC staff issued Staff Accounting Bulletin No. 118, to address the application of GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the Tax Cuts and Jobs Act (the “Tax Reform Act”). The Company has recognized the provisional tax impacts related to the revaluation of the deferred tax assets and liabilities and included these amounts in its consolidated financial statements for the year ended December 31, 2017. The ultimate impact may differ from these provisional amounts due to, among other things, additional analysis, changes in interpretations and assumptions the Company has made, additional regulatory guidance that may be issued, and actions the Company may take as a result of the Tax Reform Act. The accounting was completed when the Company’s 2017 U.S. corporate income tax return was filed in 2018, and no material differences arose as compared to provisional amounts initially reflected in the consolidated financial statements for the year ended December 31, 2017. In June 2018, the FASB issued ASU No. 2018-07, Compensation-Stock Compensation: Improvements to Nonemployee Share-Based Payment Accounting (“ASC 718”). The new standard largely aligns the accounting for share-based payment awards issued to employees and nonemployees by expanding the scope of ASC 718 to apply to nonemployee share-based transactions, as long as the transaction is not effectively a form of financing. The new guidance will be effective for the Company on January 1, 2019. The Company is currently evaluating the potential impact that this guidance may have on its consolidated financial statements. In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Requirements for Fair Value Measurement (“ASU 2018-13”). The new standard added, modified or removed disclosure requirements under Topic 820 for clarity and consistency. ASU 2018-13 is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. The Company is currently evaluating the potential impact that this guidance may have on its consolidated financial statements.

Summary of significant accoun_3

Summary of significant accounting policies (Tables)	9 Months Ended
	Sep. 30, 2018
ASU 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Schedule of impact of adoption of financial statement line items shows comparative balances under ASC 606 and ASC 605	Condensed Consolidated Statements of Operations and Comprehensive Loss Three months ended September 30, 2018 Under ASC 606 Under ASC 605 Effect of change (in thousands, except per share data) Collaboration revenue $ 2,094 $ 3,054 $ (960) Loss before income taxes (20,289) (19,329) (960) Net loss (20,289) (19,329) (960) Net loss per share, basic and diluted (0.63) (0.60) (0.03) Nine months ended September 30, 2018 Under ASC 606 Under ASC 605 Effect of change (in thousands, except per share data) Collaboration revenue $ 5,611 $ 7,267 $ (1,656) Loss before income taxes (65,937) (64,281) (1,656) Net loss (65,757) (64,101) (1,656) Net loss per share, basic and diluted (2.06) (2.00) (0.06) Condensed Consolidated Balance Sheets As of September 30, 2018 Under ASC 606 Under ASC 605 Effect of change (in thousands) Deferred revenue, current $ 20,250 $ 19,078 $ 1,172 Deferred revenue, non-current 94,804 74,390 20,414 Accumulated deficit (246,520) (224,934) (21,586) Condensed Consolidated Statements of Cash Flows Nine months ended September 30, 2018 Under ASC 606 Under ASC 605 Effect of change (in thousands) Net loss $ (65,757) $ (64,101) $ (1,656) Adjustments to reconcile net loss to net cash provided by operating activities: Deferred revenue 63,388 61,732 1,656

Fair value measurements (Tables

Fair value measurements (Tables)	9 Months Ended
	Sep. 30, 2018
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 September 30, 2018 and December 31, 2017 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) As of September 30, 2018 (in thousands) Money market funds included in cash and cash equivalents $ 41,342 $ 41,342 $ — $ — Marketable securities: U.S. Treasury notes 137,459 137,459 — — Equity securities 800 800 — — Total marketable securities $ 138,259 $ 138,259 $ — $ — Warrants to purchase equity securities 356 — 356 — Total $ 179,957 $ 179,601 $ 356 $ — As of December 31, 2017 Money market funds included in cash and cash equivalents $ 30,469 $ 30,469 $ — $ — Marketable securities: U.S. Treasury notes 137,522 137,522 — — Equity securities 1,100 1,100 — — Total marketable securities $ 138,622 $ 138,622 $ — $ — Warrants to purchase equity securities 569 — 569 — Total $ 169,660 $ 169,091 $ 569 $ —
Schedule of fair value assumptions	The assumptions utilized to value the warrants to purchase equity securities of the Company as of September 30, 2018 and December 31, 2017 are as follows: As of September 30, As of December 31, 2018 2017 Risk-free interest rate 2.9 % 2.0 % Expected dividend yield — % — % Expected term (in years) 2.9 3.7 Expected volatility 112.0 % 103.5 %

Cash, cash equivalents, and a_2

Cash, cash equivalents, and available for sale marketable securities (Tables)	9 Months Ended
	Sep. 30, 2018
Cash, cash equivalents, and available for sale marketable securities
Cash, cash equivalents and marketable securities	Cash, cash equivalents, and marketable securities included the following at September 30, 2018 and December 31, 2017: Amortized Unrealized Unrealized Fair Cost Gains Losses Value (in thousands) As of September 30, 2018 Money market funds included in cash and cash equivalents $ 41,342 $ — $ — $ 41,342 Marketable securities: U.S. Treasury notes 137,504 — 45 137,459 Equity securities 1,220 — 420 800 Total marketable securities $ 138,724 $ — $ 465 $ 138,259 Total money market funds and marketable securities $ 180,066 $ — $ 465 $ 179,601 As of December 31, 2017 Money market funds included in cash and cash equivalents $ 30,469 $ — $ — $ 30,469 Marketable securities: U.S. Treasury notes 137,560 — 38 137,522 Equity securities 1,220 — 120 1,100 Total marketable securities $ 138,780 $ — $ 158 $ 138,622 Total money market funds and marketable securities $ 169,249 $ — $ 158 $ 169,091
Reconciliation of cash, cash equivalents, and restricted cash	As of September 30, As of December 31, 2018 2017 (in thousands) Cash and cash equivalents $ 42,142 $ 31,530 Restricted cash included in deposits and other noncurrent assets 735 735 Total cash, cash equivalents, and restricted cash $ 42,877 $ 32,265

Accrued expenses (Tables)

Accrued expenses (Tables)	9 Months Ended
	Sep. 30, 2018
Accrued expenses
Schedule of accrued expenses	As of September 30, As of December 31, 2018 2017 (in thousands) Research and development costs $ 3,775 $ 5,780 Employee compensation costs 2,943 3,383 Professional services 824 1,762 Accrued goods and services 721 388 Patent costs 50 120 Other 42 64 Total $ 8,355 $ 11,497

Commitments and contingencies (

Commitments and contingencies (Tables)	9 Months Ended
	Sep. 30, 2018
Commitments and contingencies
Summary of contractual obligations as of payment due date	The following table summarizes the Company’s significant contractual obligations as of payment due date by period at September 30, 2018, related to the amended 75 Sidney Street lease and the amended 64 Sidney Street lease through December 2026: Total Minimum Lease Payments (in thousands) 2018 (remainder of year) $ 1,056 2019 4,325 2020 4,731 2021 4,863 2022 5,001 2023+ 21,924 $ 41,900
Allocable arrangement consideration allocation	Performance Obligation Amount (in thousands) Research and Development Services for: Huntington’s Program $ 14,228 Parkinson’s Program 6,040 Friedreich’s Ataxia Program 14,821 Spinal Muscular Atrophy Program 29,116 Future Program 2,239 Committee Obligations: AJSC 133 DAC 207 PD Material Right 7,855 Total $ 74,639
Schedule of allocation of transaction price	Performance Obligation Amount (in thousands) Research Services $ 34,482 First Development Option Material Right 34,518 Total $ 69,000

Stock based compensation (Table

Stock based compensation (Tables)	9 Months Ended
	Sep. 30, 2018
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: Three Months Ended Nine Months Ended September 30, September 30, 2018 2017 2018 2017 (in thousands) Research and development $ 1,176 $ 2,231 $ 3,706 $ 4,107 General and administrative 1,460 1,010 9,366 2,838 Total stock compensation expense $ 2,636 $ 3,241 $ 13,072 $ 6,945
Summary of status and changes in unvested restricted stock	A summary of the status of and changes in unvested restricted stock activity under the Company’s equity award plans for the nine months ended September 30, 2018 was as follows: Weighted Average Grant Date Fair Value Shares Per Share Unvested restricted common stock as of December 31, 2017 557,979 $ 0.70 Issued — — Vested (318,211) $ 0.84 Repurchased (2,794) $ 1.11 Unvested restricted common stock as of September 30, 2018 236,974 $ 0.51
Summary of status and changes in stock options	The following is a summary of stock option activity for the nine months ended September 30, 2018: Weighted Remaining Aggregate Average Contractual Intrinsic Exercise Life Value Shares Price (in years) (in thousands) Outstanding at December 31, 2017 3,143,566 $ 11.82 Granted 2,030,191 $ 21.54 Exercised (360,099) $ 10.17 Cancelled or forfeited (647,172) $ 16.20 Outstanding at September 30, 2018 4,166,486 $ 16.02 8.6 $ 17,613 Exercisable at September 30, 2018 1,375,760 $ 12.02 7.5 $ 10,288 Vested and expected to vest at September 30, 2018 4,166,486 $ 16.02 8.6 $ 17,613
Employees and directors stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Assumptions	The fair value of each option issued to employees and directors was estimated at the date of grant using the Black‑Scholes option pricing model with the following weighted‑average assumptions: Three Months Ended Nine Months Ended September 30, September 30, 2018 2017 2018 2017 Risk-free interest rate 2.8 % 1.8 % 2.7 % 2.0 % Expected dividend yield — % — % — % — % Expected term (in years) 6.0 6.0 6.0 6.0 Expected volatility 74.5 % 72.9 % 74.4 % 73.8 %
Non-employees stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Assumptions	The fair value of each option issued to non‑employees was estimated at each vesting and reporting date using the Black‑Scholes option pricing model. The reporting date fair value was determined using the following weighted‑average assumptions: As of September 30, 2018 2017 Risk-free interest rate 3.0 % 2.2 % Expected dividend yield — % — % Expected term (in years) 7.9 8.7 Expected volatility 73.9 % 77.5 %

Net loss per share (Tables)

Net loss per share (Tables)	9 Months Ended
	Sep. 30, 2018
Net loss per share
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 because to include them would be anti-dilutive: As of September 30, 2018 2017 Unvested restricted common stock 236,974 676,148 Shares reserved for issuance under stock plans 7,212,380 3,044,614 Total 7,449,354 3,720,762

Nature of business (Details)

Nature of business (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Nature of business
Accumulated deficit	$ (246,520)	$ (160,713)
Proceeds raised	405,000
Cash, cash equivalents, and marketable debt securities	$ 179,600

Summary of significant accoun_4

Summary of significant accounting policies (Details) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018USD ($)$ / shares	Sep. 30, 2017USD ($)$ / shares	Sep. 30, 2018USD ($)item$ / shares	Sep. 30, 2017USD ($)$ / shares	Dec. 31, 2017USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of open contracts | item			1
Condensed Consolidated Statements of Operations and Comprehensive Loss
Collaboration revenue	$ 2,094		$ 5,611
Loss before income taxes	(20,289)	$ (23,346)	(65,937)	$ (58,900)
Net loss	$ (20,289)	$ (23,346)	$ (65,757)	$ (58,869)
Net loss per share, basic and diluted | $ / shares	$ (0.63)	$ (0.89)	$ (2.06)	$ (2.27)
Condensed Consolidated Balance Sheets
Deferred revenue, current	$ 20,250		$ 20,250		$ 3,380
Deferred revenue, non-current	94,804		94,804		28,180
Accumulated deficit	(246,520)		(246,520)		$ (160,713)
Condensed Consolidated Statement of Cash Flows
Net loss			(65,757)	$ (58,869)
Adjustments to reconcile net loss to net cash provided by operating activities:
Deferred revenue			63,388	$ (3,790)
ASU 2014-09 | As recognized under Topic 605
Condensed Consolidated Statements of Operations and Comprehensive Loss
Collaboration revenue	3,054		7,267
Loss before income taxes	(19,329)		(64,281)
Net loss	$ (19,329)		$ (64,101)
Net loss per share, basic and diluted | $ / shares	$ (0.60)		$ (2)
Condensed Consolidated Balance Sheets
Deferred revenue, current	$ 19,078		$ 19,078
Deferred revenue, non-current	74,390		74,390
Accumulated deficit	(224,934)		(224,934)
Condensed Consolidated Statement of Cash Flows
Net loss			(64,101)
Adjustments to reconcile net loss to net cash provided by operating activities:
Deferred revenue			61,732
ASU 2014-09 | Effect of change
Condensed Consolidated Statements of Operations and Comprehensive Loss
Collaboration revenue	(960)		(1,656)
Loss before income taxes	(960)		(1,656)
Net loss	$ (960)		$ (1,656)
Net loss per share, basic and diluted | $ / shares	$ (0.03)		$ (0.06)
Condensed Consolidated Balance Sheets
Deferred revenue, current	$ 1,172		$ 1,172
Deferred revenue, non-current	20,414		20,414
Accumulated deficit	$ (21,586)		(21,586)
Condensed Consolidated Statement of Cash Flows
Net loss			(1,656)
Adjustments to reconcile net loss to net cash provided by operating activities:
Deferred revenue			$ 1,656

Fair value measurements - Asset

Fair value measurements - Assets measured on a recurring basis - (Details) $ in Thousands	Sep. 30, 2018USD ($)item	Dec. 31, 2017USD ($)item
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 42,142	$ 31,530
Measurement Input, Risk-Free Interest Rate
Fair value assumptions
Warrants, measurement input | item	0.029	0.020
Measurement Input, Expected Term
Fair value assumptions
Warrants, measurement input | item	2.9	3.7
Measurement Input, Expected Volatility
Fair value assumptions
Warrants, measurement input | item	1.120	1.035
Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	$ 138,259	$ 138,622
Total assets measured at fair value	179,957	169,660
Level 1 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	138,259	138,622
Total assets measured at fair value	179,601	169,091
Level 2 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	356	569
U.S. Treasury notes | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	137,459	137,522
U.S. Treasury notes | Level 1 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	137,459	137,522
Equity securities | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	800	1,100
Equity securities | Level 1 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Marketable securities	800	1,100
Warrants to purchase equity securities | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Warrants to purchase equity securities	356	569
Warrants to purchase equity securities | Level 2 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Warrants to purchase equity securities	356	569
Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	41,342	30,469
Money market funds | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	41,342	30,469
Money market funds | Level 1 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 41,342	$ 30,469

Cash, cash equivalents, and a_3

Cash, cash equivalents, and available for sale marketable securities (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost
Cash and cash equivalents	$ 42,142	$ 31,530
Total marketable securities, amortized cost	138,724	138,780
Total money market funds and marketable securities	180,066	169,249
Fair Value
Cash and cash equivalents	42,142	31,530
Total marketable securities	138,259	138,622
Total money market funds and marketable securities	179,601	169,091
Unrealized Gains and Losses
Total marketable securities, unrealized losses	465	158
Total money market funds and marketable securities	$ 465	158
Maximum
Unrealized Gains and Losses
Debt securities, contractual maturity period	1 year
Money market funds
Amortized Cost
Cash and cash equivalents	$ 41,342	30,469
Fair Value
Cash and cash equivalents	41,342	30,469
U.S. Treasury notes
Amortized Cost
Marketable securities, amortized cost	137,504	137,560
Fair Value
Marketable debt securities, fair value	137,459	137,522
Unrealized Gains and Losses
Marketable securities, unrealized losses	45	38
Equity securities
Amortized Cost
Marketable equity securities, amortized cost	1,220	1,220
Fair Value
Marketable equity securities, fair value	800	1,100
Unrealized Gains and Losses
Equity securitiess, unrealized losses	$ 420	$ 120

Cash, cash equivalents, and a_4

Cash, cash equivalents, and available for sale marketable securities - Restricted cash (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Cash, cash equivalents, and available for sale marketable securities
Cash and cash equivalents	$ 42,142	$ 31,530
Restricted cash included in deposits and other noncurrent assets	735	735
Total cash, cash equivalents, and restricted cash	$ 42,877	$ 32,265	$ 48,936	$ 37,376

Accrued expenses (Details)

Accrued expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accrued expenses
Research and development costs	$ 3,775	$ 5,780
Employee compensation costs	2,943	3,383
Professional services	824	1,762
Accrued goods and services	721	388
Patent costs	50	120
Other	42	64
Total	$ 8,355	$ 11,497

Commitments and contingencies -

Commitments and contingencies - Operating Leases - (Details) $ in Thousands	1 Months Ended
	Mar. 31, 2014item	Sep. 30, 2018USD ($)	Feb. 28, 2018USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018 (remainder of year)		$ 1,056
2,019		4,325
2,020		4,731
2,021		4,863
2,022		5,001
2023 +		21,924
Total minimum lease payments		41,900
75 Sidney Street
Number of renewal options | item	2
Leasehold improvement receivable			$ 300
Term of each renewal option	5 years
Leasehold incentive received from landlord		1,600
64 Sidney Street
Leasehold incentive received from landlord		$ 3,500

Commitments and contingencies_2

Commitments and contingencies - Significant Agreements - (Details) - Sanofi Genzyme - USD ($) $ in Millions	1 Months Ended
	Oct. 31, 2017	Feb. 28, 2015
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
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	$ 540
Regulatory approval milestone
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner	220
Regulatory approval milestone | Minimum
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
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner	50
Commercial Milestone
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner	320
Commercial Milestone | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner	50
Commercial Milestone | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner	100
Collaborative Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment received		65
Premium over fair value on temporary equity issued		5
Parkinson's Program | Regulatory and commercial milestone
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments no longer receivable	$ 105
Spinal Muscular Atrophy Program
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 program specified		30
Split Territory Program
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Option exercise payment required from collaborative partner for each program specified		$ 20
Redeemable Convertible Preferred Stock Series B | Collaborative Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Temporary equity issued (in shares)		10,000,000
Temporary equity issued		$ 30
Fair value of temporary equity issued		25
Premium over fair value on temporary equity issued		$ 5

Commitments and contingencies_3

Commitments and contingencies - Accounting Analysis - (Details) $ in Thousands	1 Months Ended			3 Months Ended	9 Months Ended
	Feb. 28, 2018USD ($)item	Sep. 30, 2016USD ($)	Feb. 28, 2015USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2018USD ($)item
Allocable arrangement consideration allocation
Committee Obligations: Alliance Joint Steering Committee ("AJSC")			$ 133
Committee Obligations: Development Advisory Committee ("DAC")			207
PD Material Right			7,855
Total allocable arrangement consideration allocations.			74,639
Huntington's Program
Allocable arrangement consideration allocation
Research and development services			14,228
Parkinson's Program
Allocable arrangement consideration allocation
Research and development services			6,040
Friedreich's Ataxia Program
Allocable arrangement consideration allocation
Research and development services			14,821
Spinal Muscular Atrophy Program
Allocable arrangement consideration allocation
Research and development services			29,116
Future Program
Allocable arrangement consideration allocation
Research and development services			2,239
Sanofi Genzyme | Collaborative Arrangement
Allocable arrangement consideration
Collaborative Arrangement Nonredeemable Up Front Fee Received			65,000
Additional consideration, premium over fair value on temporary equity issued			5,000
In-kind services and other funding to be provided by collaborative partner			4,600
Total allocable arrangement consideration			74,600
Allocable arrangement consideration allocation
Additional option exercise payments | item					0
Income Statement Related Disclosures [Abstract]
Revenue recognized				$ 100	$ 1,300
Deferred revenue				50,400	$ 50,400
Research options | item					1
MRIC
Income Statement Related Disclosures [Abstract]
Payments for shares		$ 2,000
AbbVie | Collaborative Arrangement
Allocable arrangement consideration
Collaborative Arrangement Nonredeemable Up Front Fee Received	$ 69,000
Allocable arrangement consideration allocation
Research and development services					$ 34,482
First Development Option Material Right					34,518
Total allocable arrangement consideration allocations.					69,000
Income Statement Related Disclosures [Abstract]
Revenue recognized				2,000	4,300
Deferred revenue				$ 64,700	$ 64,700
Number of development options | item	1
Redeemable Convertible Preferred Stock Series B | Sanofi Genzyme | Collaborative Arrangement
Allocable arrangement consideration
Additional consideration, premium over fair value on temporary equity issued			$ 5,000

Commitments and contingencies_4

Commitments and contingencies - AbbVie Collaboration Agreement - (Details) - Collaborative Arrangement $ in Millions	1 Months Ended		3 Months Ended	9 Months Ended
	Feb. 28, 2018USD ($)item	Feb. 28, 2015USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2018USD ($)
AbbVie
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research compounds number | item	2
Non-refundable upfront payment received	$ 69
First Research Compound payment	80
Two additional research compounds payment to be received for each	$ 30
Number of additional research compounds | item	2
One-time payment upon exercise of license option	$ 75
Milestone payments to be received for licensed compound in case Alzheimer disease indication	550
Milestone payments to be received for licensed compound in case first indication other than Alzheimer disease	230
Milestone payments to be received for licensed compound for subsequent non-Alzheimer disease indication	$ 115
Number of development options | item	1
Number of research antibodies | item	1
The number of Licensed Products | item	1
Deferred revenue			$ 64.7	$ 64.7
Revenue recognized			2	4.3
AbbVie | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of antibodies encoded by virus | item	5
Antibodies to include in Research Period | item	3
Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-refundable upfront payment received		$ 65
Deferred revenue			50.4	50.4
Revenue recognized			$ 0.1	$ 1.3

Commitments and contingencies_5

Commitments and contingencies - Other Agreements - (Details) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016USD ($)multiple	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Dec. 31, 2017USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Expenses incurred		$ 23,241	$ 24,503	$ 73,545	$ 63,305
Non-profit organization agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner						$ 1,000
Deferred revenue		1,000		1,000
Maximum funding	$ 4,000
Funding multiple | multiple	2.6
MRIC
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Expenses incurred		$ 100		$ 200

Redeemable convertible prefer_2

Redeemable convertible preferred stock (Details) - shares	Sep. 30, 2018	Dec. 31, 2017
Redeemable convertible preferred stock
Preferred stock, authorized	5,000,000	5,000,000

Stock based compensation (Detai

Stock based compensation (Details) $ / shares in Units, $ in Thousands	Jul. 16, 2018$ / sharesshares	Jan. 01, 2018shares	Jan. 01, 2017shares	Jan. 01, 2016shares	Oct. 31, 2015shares	Jan. 31, 2014$ / sharesshares	Sep. 30, 2018USD ($)item$ / sharesshares	Sep. 30, 2017USD ($)shares	Sep. 30, 2018USD ($)item$ / sharesshares	Sep. 30, 2017USD ($)shares	Dec. 31, 2016item	Dec. 31, 2017$ / shares	Aug. 31, 2015shares	Apr. 30, 2015shares	Aug. 31, 2014shares	Apr. 30, 2014shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $							$ 2,636	$ 3,241	$ 13,072	$ 6,945
Stock options granted (in shares)									2,030,191
Exercise price | $ / shares							$ 16.02		$ 16.02			$ 11.82
2014 Stock Option and Grant Plan (2014 Plan)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for stock-based compensation plan (in shares)					2,998,823	823,529							2,669,411	2,047,058	2,000,000	1,411,764
2015 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares authorized for issuance		321,300	267,658	267,492
Number of shares authorized for stock-based compensation plan (in shares)					262,362
Period to purchase shares									6 months
Increase in ESSP shares (as a percent)									1.00%
2015 Employee Stock Purchase Plan | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Payroll deduction percentage					1.00%
2015 Employee Stock Purchase Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Payroll deduction percentage					10.00%
2015 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)					1,311,812		2,055,980		2,055,980
Percentage limit of issued and outstanding shares up to which additional shares may be issued									4.00%
Common stock shares authorized for issuance		1,285,200	1,070,635	1,069,971
Unvested restricted stock awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock-based compensation expense | $							$ 100		$ 100
Performance based milestone awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Probable performance milestones | item											1
Performance milestones | item											3
Stock-based compensation | $							$ 100	300	300	900
Founders restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $									$ 0
Restricted stock granted (in shares)						1,188,233
Restricted stock granted, original issued price (in dollars per share) | $ / shares						$ 0.0425
Number of awards | item							2		2
Founders restricted stock | Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock granted (in shares)						835,292
Founders restricted stock | Vesting period, tranche two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock granted (in shares)						352,941
Founders restricted stock | Minimum | Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period						1 year
Founders restricted stock | Maximum | Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period						4 years
Employees and directors stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $							$ 2,500	1,400	$ 12,300	3,800
Employees and directors stock options | 2015 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New stock options issued							846,000		2,030,191
Non-employees stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $							$ 100	$ 100	$ 300	$ 300
Stock options granted (in shares)							0	20,000	0	20,000
Non-employees stock options | 2015 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New stock options issued							0		0
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $							$ 1,176	$ 2,231	$ 3,706	$ 4,107
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $							$ 1,460	$ 1,010	$ 9,366	$ 2,838
Chief Executive Officer | Non-statutory stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for stock-based compensation plan (in shares)	650,000
Exercise price | $ / shares	$ 18.03
Vesting period	4 years
Chief Executive Officer | Non-statutory stock options | Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	12 months
Options vested (as a percent)	25.00%
Chief Executive Officer | Non-statutory stock options | Vesting period, tranche two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	36 months
Options vested (as a percent)	75.00%

Stock based compensation - Rest

Stock based compensation - Restricted Stock - (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Shares
Balance, beginning (in shares)			557,979
Vested (in shares)			(318,211)
Repurchased (in shares)			(2,794)
Balance, ending (in shares)	236,974		236,974
Weighted Average Grant Date Fair Value Per Share
Balance, beginning (in dollars per share)			$ 0.70
Vested (in dollars per share)			0.84
Repurchased (in dollars per share)			1.11
Balance, ending (in dollars per share)	$ 0.51		$ 0.51
Restricted stock disclosures
Stock-based compensation	$ 2,636,000	$ 3,241,000	$ 13,072,000	$ 6,945,000
Unvested restricted stock awards
Restricted stock disclosures
Unrecognized stock-based compensation expense	100,000		$ 100,000
Remaining weighted-average remaining vesting period			2 months 12 days
Employees restricted stock
Restricted stock disclosures
Stock-based compensation	4,600	100,000	$ 100,000	400,000
Non-employee consultants restricted stock
Restricted stock disclosures
Stock-based compensation	$ 18,500	$ 1,600,000	$ 400,000	$ 2,300,000

Stock based compensation - Stoc

Stock based compensation - Stock Options - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding, beginning balance (in shares)			3,143,566
Granted (in shares)			2,030,191
Exercised (in shares)			(360,099)
Cancelled or forfeited (in shares)			(647,172)
Outstanding, ending balance (in shares)	4,166,486		4,166,486
Exercisable (in shares)	1,375,760		1,375,760
Vested and expected to vest (in shares)	4,166,486		4,166,486
Weighted Average Exercise Price
Outstanding (in dollars per share)			$ 11.82
Granted (in dollars per share)			21.54
Exercised (in dollars per share)			10.17
Cancelled or forfeited (in dollars per share)			16.20
Outstanding (in dollars per share)	$ 16.02		16.02
Exercisable (in dollars per share)	12.02		12.02
Vested and expected to vest (in dollars per share)	$ 16.02		$ 16.02
Remaining Contractual Life
Outstanding			8 years 7 months 6 days
Exercisable			7 years 6 months
Vested and expected to vest			8 years 7 months 6 days
Aggregate Intrinsic Value
Outstanding	$ 17,613		$ 17,613
Exercisable	10,288		10,288
Vested and expected to vest	17,613		17,613
Options disclosures
Stock-based compensation	$ 2,636	$ 3,241	$ 13,072	$ 6,945
Employees and directors stock options
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 12.20	$ 9.43	$ 14.37	$ 7.88
Stock-based compensation	$ 2,500	$ 1,400	$ 12,300	$ 3,800
Non-employees stock options
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Granted (in shares)	0	20,000	0	20,000
Options disclosures
Stock-based compensation	$ 100	$ 100	$ 300	$ 300

Stock based compensation - Blac

Stock based compensation - Black-Scholes options pricing model - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Options disclosures
Stock-based compensation	$ 2,636	$ 3,241	$ 13,072	$ 6,945
Stock options granted (in shares)			2,030,191
Employees and directors stock options
Fair value assumptions
Risk-free interest rate	2.80%	1.80%	2.70%	2.00%
Expected term	6 years	6 years	6 years	6 years
Expected volatility	74.50%	72.90%	74.40%	73.80%
Options disclosures
Weighted average fair value of grants (in dollars per share)	$ 12.20	$ 9.43	$ 14.37	$ 7.88
Stock-based compensation	$ 2,500	$ 1,400	$ 12,300	$ 3,800
Non-employees stock options
Fair value assumptions
Risk-free interest rate			3.00%	2.20%
Expected term			7 years 10 months 24 days	8 years 8 months 12 days
Expected volatility			73.90%	77.50%
Options disclosures
Stock-based compensation	$ 100	$ 100	$ 300	$ 300
Stock options granted (in shares)	0	20,000	0	20,000
Unvested stock options
Options disclosures
Unrecognized stock-based compensation expense	$ 27,700		$ 27,700
Remaining weighted-average remaining vesting period			2 years 10 months 24 days

Income taxes (Details)

Income taxes (Details) $ in Thousands	3 Months Ended
	Sep. 30, 2017USD ($)
Income taxes
Tax expense (benefit) recognized in OCI	$ (53)

Net loss per share (Details)

Net loss per share (Details) - shares	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	7,449,354	3,720,762
Unvested restricted stock awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	236,974	676,148
Shares reserved for issuance under stock plans
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	7,212,380	3,044,614