Dimension Therapeutics (DMTX) 10-Q 4 Dec 15 2015 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2015	Dec. 04, 2015
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2015
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	Q3
Trading Symbol	DMTX
Entity Registrant Name	Dimension Therapeutics, Inc.
Entity Central Index Key	1,592,288
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		24,996,921

Condensed Balance Sheets

Condensed Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 69,245	$ 17,913
Accounts receivable	2,092	1,974
Prepaid expenses and other current assets	1,694	404
Total current assets	73,031	20,291
Property and equipment, net	3,506	1,780
Deferred offering costs	2,508
Other assets		62
Total assets	79,045	22,133
Current liabilities:
Accounts payable	775	675
Accrued expenses and other current liabilities	4,449	1,080
Amounts due to related parties	21	750
Deferred revenue	6,335	5,129
Notes payable	574	358
Total current liabilities	12,154	7,992
Deferred revenue, net of current portion	16,380	17,513
Notes payable, net of discount and current portion	903	1,166
Other liabilities	64
Total liabilities	$ 29,501	$ 26,671
Commitments and contingencies (Note 11)
Convertible preferred stock (Series A and B), $0.0001 par value; 44,683,824 shares authorized as of September 30, 2015 and 45,000,000 shares authorized as of December 31, 2014; 44,683,824 and 10,000,000 shares issued and outstanding as of September 30, 2015 and December 31, 2014, respectively; aggregate liquidation preference of $89,500 and $10,000 as of September 30, 2015 and December 31, 2014	$ 88,895	$ 9,653
Stockholders' deficit:
Common stock, $0.0001 par value; 68,000,000 shares authorized as of September 30, 2015 and 65,700,000 shares authorized as of December 31, 2014; 4,105,178 and 4,056,818 shares issued and outstanding as of September 30, 2015 and December 31, 2014, respectively	0	0
Additional paid-in capital	2,815	1,947
Accumulated deficit	(42,166)	(16,138)
Total stockholders' deficit	(39,351)	(14,191)
Total liabilities, convertible preferred stock and stockholders' deficit	$ 79,045	$ 22,133

Condensed Balance Sheets (Paren

Condensed Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Convertible preferred stock (Series A and B), par value	$ 0.0001	$ 0.0001
Convertible preferred stock (Series A and B), shares authorized	44,683,824	45,000,000
Convertible preferred stock (Series A and B), shares issued	44,683,824	10,000,000
Convertible preferred stock (Series A and B), shares outstanding	44,683,824	10,000,000
Convertible preferred stock (Series A and B), aggregate liquidation preference	$ 89,500	$ 10,000
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	68,000,000	65,700,000
Common stock, shares issued	4,105,178	4,056,818
Common stock, shares outstanding	4,105,178	4,056,818

Condensed Statements of Operati

Condensed Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Statement [Abstract]
Revenue	$ 2,061	$ 1,184	$ 5,397	$ 1,343
Operating expenses:
Research and development	10,372	3,217	25,238	8,157
General and administrative	2,334	451	6,115	1,536
Total operating expenses	12,706	3,668	31,353	9,693
Loss from operations	(10,645)	(2,484)	(25,956)	(8,350)
Interest income (expense), net	(22)		(72)
Net loss and comprehensive loss	(10,667)	(2,484)	(26,028)	(8,350)
Accretion of convertible preferred stock to redemption value		(18)	(23)	(52)
Net loss attributable to common stockholders	$ (10,667)	$ (2,502)	$ (26,051)	$ (8,402)
Net loss per share attributable to common stockholders - basic and diluted	$ (2.73)	$ (0.70)	$ (6.71)	$ (2.35)
Weighted average common shares outstanding - basic and diluted	3,907,196	3,585,603	3,880,779	3,582,213

Condensed Statements of Cash Fl

Condensed Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Cash flows from operating activities:
Net loss	$ (26,028)	$ (8,350)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Consulting services paid in common stock		37
Depreciation and amortization expense	507	77
Stock-based compensation expense	886	49
Non-cash interest expense	7	1
Changes in operating assets and liabilities:
Accounts receivable	(118)	(1,236)
Prepaid expenses and other current assets	(1,290)	(187)
Accrued expenses and other current liabilities and amounts due to related parties	1,633	(588)
Accounts payable	54	651
Deferred revenue	72	19,948
Other liabilities	64
Net cash (used in) provided by operating activities	(24,213)	10,402
Cash flows from investing activities:
Purchases of property and equipment	(2,186)	(1,198)
Lease deposits	62	(62)
Net cash used in investing activities	(2,124)	(1,260)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs	79,219	4,875
Proceeds from exercise of common stock options	5
Proceeds from issuance of notes payable and common stock warrant, net of issuance costs	194	1,127
Payments of initial public offering costs	(1,501)
Repayment of notes payable	(248)
Net cash provided by financing activities	77,669	6,002
Net increase in cash and cash equivalents	51,332	15,144
Cash and cash equivalents at beginning of period	17,913	4,507
Cash and cash equivalents at end of period	69,245	19,651
Supplemental disclosure of cash flow information:
Interest paid	68
Supplemental disclosure of non-cash investing and financing activities:
Accretion of convertible preferred stock to redemption value	23	52
Issuance of common stock warrant in connection with notes payable		5
Purchases of property and equipment included in accounts payable and accrued expenses	47	$ 322
Deferred offering costs included in accounts payable and accrued expenses	$ 1,007

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2015
Accounting Policies [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Dimension Therapeutics, Inc. (the “Company”) was incorporated in Delaware on June 20, 2013. The Company is a gene therapy platform company focused on discovering and developing new therapeutic products for people living with rare diseases associated with the liver and caused by genetic mutations. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations and ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. As of September 30, 2015 and December 31, 2014, the Company had an accumulated deficit of $42,166 and $16,138, respectively. The Company expects to continue to generate operating losses in the foreseeable future. On October 27, 2015 the Company completed an initial public offering (“IPO”) of its common stock, which resulted in the issuance and sale of 5,500,000 shares of its common stock at a public offering price of $13.00 per share, resulting in net proceeds of approximately $63,445 after deducting underwriting discounts and other estimated offering costs. Upon the closing of the IPO, all outstanding shares of the Company’s Convertible Preferred Stock were automatically converted into 15,286,968 shares of the Company’s common stock. On November 25, 2015, the underwriters of the Company’s IPO exercised their over-allotment option to purchase an additional 104,775 shares of common stock at the initial public offering price of $13.00 per share, resulting in net proceeds of approximately $1,250, after deducting underwriting discounts and other estimated offering costs. The Company may seek additional funding through public or private financings, debt financing, collaboration agreements or government grants. The Company may not be able to obtain financing on acceptable terms, or at all, and the Company may not be able to enter into additional collaboration arrangements or obtain government grants. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate its research and development programs, product portfolio expansion or commercialization efforts, which could adversely affect its business prospects. Although management continues to pursue these plans, there is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations, if at all. The condensed financial statements are unaudited and have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”).

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2015
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies The accounting policies followed in the preparation of the interim condensed financial statements are consistent in all material respects with those presented in Note 2 to the financial statements for the year ended December 31, 2014 included in the Company’s final prospectus dated October 21, 2015, filed pursuant to Rule 424(b)(4) under the Securities Act of 1933 on October 22, 2015. Unaudited Interim Financial Information The accompanying condensed balance sheet as of September 30, 2015, the condensed statements of operations and comprehensive loss for the three and nine months ended September 30, 2015 and 2014 and of condensed cash flows for the nine months ended September 30, 2015 and 2014 are unaudited. The unaudited interim condensed financial statements have been prepared on the same basis as the audited annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of September 30, 2015 and the results of its operations and its cash flows for the nine months ended September 30, 2015 and 2014. The financial data and other information disclosed in the notes related to the three and nine months ended September 30, 2015 and 2014 are unaudited. The results for the nine months ended September 30, 2015 are not necessarily indicative of results to be expected for the year ending December 31, 2015, any other interim periods, or any future year or period. The unaudited interim financial statements of the Company included herein have been prepared pursuant to the rules and regulations of the SEC. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted from this report, as is permitted by such rules and regulations. These unaudited condensed financial statements should be read in conjunction with the audited financial statements and the notes thereto for the year ended December 31, 2014 included in the Company’s final prospectus dated October 21, 2015, filed pursuant to Rule 424(b)(4) under the Securities Act of 1933 on October 22, 2015. Recently Issued Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date In August 2014, the FASB issued ASU No. 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern In April 2015, the FASB issued ASU No. 2015-03, Interest — Imputation of Interest Subtopic 835-30 In November 2015, the FASB issued ASU No. 2015-17, Income Taxes (Subt opic 740)

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	9 Months Ended
	Sep. 30, 2015
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets and Liabilities	3. Fair Value of Financial Assets and Liabilities The following tables present information about the Company’s financial assets measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values: Fair Value Measurements as of September 30, 2015 using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ 69,245 $ — $ — $ 69,245 $ 69,245 $ — $ — $ 69,245 Fair Value Measurements as of December 31, 2014 using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ 17,836 $ — $ — $ 17,836 $ 17,836 $ — $ — $ 17,836 As of September 30, 2015 and December 31, 2014, the Company’s cash equivalents, which were invested in a money market fund, were valued based on Level 1 inputs. During the nine months ended September 30, 2015 and the year ended December 31, 2014, there were no transfers between Level 1, Level 2 and Level 3.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	9 Months Ended
	Sep. 30, 2015
Payables and Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	4. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of the following: September 30, 2015 December 31, 2014 Payroll and payroll-related costs $ 1,307 $ 469 External research and development services 1,582 362 Professional fees 1,507 159 Other 53 90 $ 4,449 $ 1,080

Notes Payable

Notes Payable	9 Months Ended
	Sep. 30, 2015
Debt Disclosure [Abstract]
Notes Payable	5. Notes Payable In January 2015, the Company borrowed the remaining $194 available under the equipment line provided for in its loan and security agreement. During the three and nine months ended September 30, 2015, the Company made scheduled principal repayments totaling $140 and $248, respectively. Borrowings under the loan and security agreement are collateralized by substantially all of the Company’s assets, except for its intellectual property. Accretion of debt discount recorded as additional interest expense was $2 and $7 for the three and nine months ended September 30, 2015. As of September 30, 2015, the unamortized debt discount was $25. As of September 30, 2015, annual principal repayment requirements under the loan and security agreement were $146 for the remainder of the year ending December 31, 2015, $583 during the year ending December 31, 2016, $583 during the year ending December 31, 2017 and $190 during the year ending December 31, 2018.

Collaboration Agreement

Collaboration Agreement	9 Months Ended
	Sep. 30, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaboration Agreement	6. Collaboration Agreement Bayer HealthCare LLC In June 2014, the Company entered into a research and development collaboration and license agreement with Bayer for the development and commercialization of a gene therapy for the treatment of hemophilia A (the “collaboration agreement”). Under the agreement, the Company granted to Bayer an exclusive license to develop and commercialize one or more novel gene therapies for hemophilia A and a right of first notice for gene therapy treatments for hemophilia B. The Company is responsible for all research and development activities for those biologic therapeutics under the agreement prior to completion of clinical proof-of-concept (“POC”) trials, with Bayer funding the Company’s efforts and providing consulting and technological support. Upon the successful demonstration of clinical POC, the agreement requires that Bayer use commercially reasonable efforts to conduct later-stage development and commercialization of gene therapy products for treatment of hemophilia A for such therapy. Bayer will have worldwide rights to commercialize the potential future product. Financial Terms Under the terms of the agreement with Bayer, the Company received a nonrefundable, noncreditable upfront license payment of $20,000 in June 2014 and is eligible to receive development and commercialization milestone payments of up to $232,000, as well as tiered royalty payments ranging in the high single-digit to low double-digit percentages, not exceeding the mid-teens, of net sales of commercialized products resulting from the collaboration, as defined in the agreement with Bayer. Bayer will fund certain research and development services performed by the Company during the research term and will reimburse the Company for all project costs, including any third-party costs, in the performance of its obligations under the annual research plan and in accordance with the mutually agreed upon research budget. The Company determined that the deliverables under its agreement with Bayer include (i) research services to be provided over the research term, (ii) a development and commercialization license and (iii) the Company’s participation in a Joint Steering Committee (“JSC”) and a Joint Research and Development Committee (“JRDC”) to be provided over the research term. The Company determined that the development and commercialization license and involvement in the JSC and the JRDC do not have standalone value to Bayer and, therefore, are not separable from the delivery of the research services. Therefore, all deliverables under the agreement have been combined and accounted for as a single unit of accounting. Accordingly, the upfront license payment and research payments are being recognized by the Company as revenue on a straight-line basis over the estimated performance period, which approximates the 54-month research term of the agreement with Bayer that commenced in June 2014. As future research payments are earned, the Company will recognize as revenue the portion of payments equal to the percentage of the elapsed research term to the total estimated research term, with the remaining portion of consideration received being recognized over the remaining estimated performance period on a straight-line basis. The Company concluded at the outset of the arrangement that none of the future milestone payments included in the arrangement qualified as substantive milestones. Any future milestone payments will be recognized, along with the other arrangement consideration, over the remaining estimated period of performance, if any, beginning at the time a milestone payment is earned, with a cumulative catch-up being recognized for the elapsed portion of the estimated research term. As of September 30, 2015, the Company had not earned any milestone or royalty payments. Revenue recognized under the collaboration agreement with Bayer consisted of the following: Three Months Ended September 30, Nine Months Ended September 30, 2015 2014 2015 2014 Revenue: Bayer Collaboration $ 2,061 $ 1,184 $ 5,397 $ 1,343 $ 2,061 $ 1,184 $ 5,397 $ 1,343 During the three and nine months ended September 30, 2015 and 2014, research payments earned by the Company under the collaboration agreement totaled $2,092, $1,290, $5,470 and $1,290, respectively. The costs incurred by the Company related to the research activities of the collaboration agreement are recorded as research and development expense in the condensed statement of operations and comprehensive loss. As of September 30, 2015, deferred revenue related to the collaboration agreement with Bayer totaled $22,715, which related to the upfront license payment, research services payments and to advance payments for future research services. Term and Termination The agreement with Bayer expires on a licensed treatment-by-licensed treatment and country-by-country basis until the later of ten years from the date of first commercial sale or when patent claims have expired, lapsed, been abandoned or been invalidated in such country. The Company and Bayer, through the JSC, may mutually agree to terminate the collaboration early in the event clinical development is ended as required by a regulatory authority or in light of data from any studies conducted prior the POC trial. Bayer has the unilateral right to terminate the agreement in its entirety or with respect to one or more country by providing written notice to the Company. Bayer may also terminate the agreement with notice after two unsuccessful attempts to demonstrate clinical POC in a Phase I human POC trial. Bayer may further terminate the agreement with notice in the event that, following the POC trial, there is a material safety issue with respect to a licensed product. Reciprocal termination rights under the agreement include termination for breach and termination for bankruptcy.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2015
Related Party Transactions [Abstract]
Related Party Transactions	7. Related Party Transactions Related party expenses consisted of the following: Three Months Ended September 30, Nine Months Ended September 30, 2015 2014 2015 2014 Related Parties: REGENX - R&D Expense $ 1,000 $ 1,445 $ 2,000 $ 4,881 University of Pennsylvania - R&D Expense 1,498 — 4,158 — Fidelity Biosciences - G&A Expense — 14 — 42 $ 2,498 $ 1,459 $ 6,158 $ 4,923 Cash payments made to REGENXBIO, Inc. (“REGENX”) during the three months ended September 30, 2015 and 2014 were $1,000 and $2,083 and during the nine months ended September 30, 2015 and 2014 were $2,750 and $5,896, respectively. Amounts due to REGENX by the Company were $0 and $750 as of September 30, 2015 and December 31, 2014, respectively. Cash payments made to the University of Pennsylvania during the three months ended September 30, 2015 and 2014 were $1,000 and $0 and during the nine months ended September 30, 2015 and 2014 were $4,137 and $0, respectively. Amounts due to University of Pennsylvania by the Company were $21 and $0 as of September 30, 2015 and December 31, 2014, respectively. There were no cash payments made to Fidelity Biosciences during the three months ended September 30, 2015 and 2014. Cash payments made to Fidelity Biosciences were $1 and $38 and during the nine months ended September 30, 2015 and 2014, respectively. There were no amounts due to Fidelity Biosciences by the Company as of September 30, 2015 or December 31, 2014.

Common and Convertible Preferre

Common and Convertible Preferred Stock	9 Months Ended
	Sep. 30, 2015
Equity [Abstract]
Common and Convertible Preferred Stock	8. Common and Convertible Preferred Stock On February 2, 2015, the Company issued 14,500,000 shares of Series A Preferred Stock, par value $0.0001 per share, at a per share price of $1.00 for aggregate gross proceeds of $14,500, net of issuance costs of $20. In February 2015, the Stock Purchase Agreement was further amended to remove the investors’ obligations to provide funding in the second and third tranches to the Company upon achieving specified milestones and to remove the investors’ options to purchase additional shares in non-milestone closings. On April 20, 2015, the Company issued 20,183,824 shares of Series B Preferred Stock, par value $0.0001 per share, at a per share price of $3.2204 for aggregate gross proceeds of $65,000, net of issuance costs of $261. As of September 30, 2015, the Company’s certificate of incorporation, as amended and restated, authorized the Company to issue 68,000,000 shares of $0.0001 par value common stock. On October 27, 2015, in connection with the closing of the IPO, the Company effected its 4 th

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	9. Stock-Based Compensation 2013 Stock Plan The Company’s 2013 Stock Plan, as amended, (the “2013 Plan”) provides for the Company to sell or issue common stock or restricted common stock, or to grant incentive stock options or nonqualified stock options for the purchase of common stock, to employees, members of the board of directors and consultants of the Company. The 2013 Plan is administered by the board of directors, or at the discretion of the board of directors, by a committee of the board. The exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or their committee if so delegated, except that the exercise price per share of stock options may not be less than 100% of the fair market value of the share of common stock on the date of grant and the term of stock option may not be greater than ten years. Vesting periods are determined at the discretion of the board of directors. Stock options granted to employees and directors typically vest over four years. Stock options granted to non-employees typically vest over periods ranging from two years to four years, depending on the period during which the services are provided. The Company measures and records the value of options granted to non-employees over the period of time services are provided and, as such, unvested portions are subject to remeasurement at subsequent reporting periods. During the nine months ended September 30, 2015, the Company granted to employees and directors 35,788 shares of restricted common stock and options to purchase 1,938,705 shares of common stock under the 2013 Plan. The total number of shares that may be issued under the 2013 Plan was 3,520,396 shares as of September 30, 2015, of which 54,510 shares remained available for future grant. 2015 Stock Option and Incentive Plan On September 2, 2015, the Company’s stockholders approved the 2015 Stock Option and Incentive Plan (the “2015 Plan”), which became effective October 21, 2015. The 2015 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock units, restricted stock awards, performance share awards, cash-based awards and other stock-based awards. 1,422,886 shares were initially reserved for issuance under the 2015 Plan. The number of shares of common stock that may be issued under the 2015 Plan will automatically increase on each January 1, beginning on January 1, 2016 and ending on January 1, 2020, equal to the lesser of (i) 4% of the shares of the Company’s common stock outstanding on the immediately preceding December 31 or (ii) an amount determined by the Company’s board of directors or the compensation committee of the board of directors. The shares of common stock underlying any awards that are forfeited, canceled, repurchased or are otherwise terminated by the Company under the 2015 Plan and the 2013 Plan will be added back to the shares of common stock available for issuance under the 2015 Plan. 2015 Employee Stock Purchase Plan On September 2, 2015, the Company’s stockholders approved the 2015 Employee Stock Purchase Plan (the “ESPP”), which became effective October 27, 2015. A total of 256,585 shares of common stock were reserved for issuance under this plan. In addition, the number of shares of common stock that may be issued under the ESPP will automatically increase on each January 1, beginning on January 1, 2017 and ending on January 1, 2021, equal to the lesser of (i) 256,585 shares of common stock, (ii) 1% of the Company’s shares of common stock outstanding on the immediately preceding December 31 or (iii) an amount determined by the Company’s board of directors or the compensation committee of the board of directors. During the nine months ended September 30, 2015, the Company did not grant to non-employees any stock-based awards. Stock Option Valuation The assumptions that the Company used to determine the fair value of the stock options granted to employees and directors during the nine months ended September 30, 2015 were as follows, presented on a weighted average basis: 1.76% risk-free interest rate; 6.00-year expected term; 72.98% expected volatility; and no dividend yield. Stock Options The following table summarizes the Company’s stock option activity since December 31, 2014: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) Outstanding as of December 31, 2014 1,104,172 $ 0.53 9.7 $ 67 Granted 1,938,705 $ 4.06 Exercised (12,572 ) $ 0.38 Cancelled or forfeited (8,981 ) $ 0.38 Outstanding as of September 30, 2015 3,021,324 $ 2.80 9.4 $ 22,359 Options vested and expected to vest as of September 30, 2015 3,021,324 $ 2.80 9.4 $ 22,359 Options exercisable as of September 30, 2015 311,081 $ 1.24 9.0 $ 2,788 The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock at September 30, 2015. The weighted average grant-date fair value of stock options granted during the three and nine months ended September 30, 2015 was $4.91 and $4.06 per share, respectively. As of September 30, 2015, there were outstanding unvested service-based stock options held by non-employees for the purchase of 3,934 shares of common stock. Restricted Common Stock The Company has granted restricted common stock with time-based vesting conditions. Unvested shares of restricted common stock may not be sold or transferred by the holder. These restrictions lapse according to the time-based vesting conditions of each award. The following table summarizes the Company’s restricted common stock activity: Number of Shares Weighted Average Grant Date Fair Value Unvested restricted common stock at December 31, 2014 210,287 $ 0.53 Issued 35,788 $ 3.57 Vested (72,067 ) $ 1.37 Unvested restricted common stock at September 30, 2015 174,008 $ 0.86 The total fair value of restricted common stock vested during the nine months ended September 30, 2015 was $295. Stock-Based Compensation Stock-based compensation expense related to stock options and restricted common stock was classified in the condensed statements of operations and comprehensive loss as follows: Three Months Ended September 30, Nine Months Ended September 30, 2015 2014 2015 2014 Research and development $ 318 $ 14 $ 487 $ 32 General and administrative 232 9 399 17 $ 550 $ 23 $ 886 $ 49 As of September 30, 2015, total unrecognized compensation cost related to the unvested awards was $5,108, which is expected to be recognized over a weighted average period of 3.5 years.

Net Loss per Share

Net Loss per Share	9 Months Ended
	Sep. 30, 2015
Earnings Per Share [Abstract]
Net Loss per Share	10. Net Loss per Share Basic and diluted net loss per share attributable to common stockholders was calculated as follows: Three Months Ended September 30, Nine Months Ended September 30, 2015 2014 2015 2014 Numerator: Net loss $ (10,667 ) $ (2,484 ) $ (26,028 ) $ (8,350 ) Accretion of convertible preferred stock to redemption value — (18 ) (23 ) (52 ) Net loss attributable to common stockholders $ (10,667 ) $ (2,502 ) $ (26,051 ) $ (8,402 ) Denominator: Weighted average common shares outstanding 4,093,836 4,056,818 4,077,551 3,950,468 Less: Weighted average shares of unvested restricted common stock outstanding (186,640 ) (471,215 ) (196,772 ) (368,255 ) Weighted average common shares outstanding used in calculating net loss per share attributable to common stockholders 3,907,196 3,585,603 3,880,779 3,582,213 Net loss per share attributable to common stockholders $ (2.73 ) $ (0.70 ) $ (6.71 ) $ (2.35 ) The Company’s potential dilutive securities, which include convertible preferred stock, stock options, unvested restricted common stock and a warrant to purchase common stock, have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an antidilutive effect: Three and Nine Months Ended September 30, 2015 2014 Convertible preferred stock (as converted to common stock) 15,286,968 3,421,142 Options to purchase common stock 3,021,324 811,669 Unvested restricted common stock 174,008 469,211 Warrant for the purchase of common stock 11,973 11,973 18,494,273 4,713,995

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	11. Commitments and Contingencies Leases The Company leases office and laboratory space under various operating lease agreements for its Cambridge, MA location that, as amended, expire between March 2015 and January 2018. The Company recorded rent expense of $171 and $71 during the three months ended September 30, 2015 and 2014 and $563 and $157 during the nine months ended September 30, 2015 and 2014, respectively. On November 2, 2015, the Company entered into an operating lease agreement with ARE-MA Region No. 20, LLC (“ARE”) for approximately 17,500 square feet of laboratory and office space located at 19 Presidential Way, Woburn, Massachusetts. Rent payments will commence on February 15, 2016 and will continue for a term of 5 years from substantial completion of the building improvements, which is currently estimated to be on or around April 1, 2016. Annual rent under the lease, exclusive of operating expenses and real estate taxes, will be approximately $0.4 million for the first year, with annual increases of $1.00 per square foot each year thereafter. Under the terms of the lease, ARE will contribute $35.00 per rentable square foot towards a tenant improvement allowance, which is included in the base rent set forth in the lease. The ARE lease contains customary provisions allowing ARE to terminate our lease should we fail to remedy any breach of our obligations within specified time periods, or upon our bankruptcy or insolvency. The following table summarizes the future minimum lease payments under the Cambridge and Woburn operating leases: Year Ending December 31, 2015 (remainder of year) $ 173 2016 1,111 2017 1,203 2018 548 2019 503 2020 521 2021 131 Total $ 4,190 The Company is further responsible for its pro rata share of operating expenses, real estate taxes and utilities of the facility housing the leased office and laboratory space. License Agreements The Company has entered into two license agreements with REGENX under which it is obligated to make contingent payments. See Notes 8 and 14 to the financial statements included in the Company’s final prospectus dated October 21, 2015, filed pursuant to Rule 424(b)(4) under the Securities Act of 1933 on October 22, 2015 for further information. Research and Development Agreement In June 2015, the Company entered into a three-year cooperative research and development agreement with the Eunice Kennedy Shriver National Institute of Child Health and Human Development, an institute of the U.S. National Institutes of Health, to conduct on behalf of the Company a non-clinical study to investigate the effectiveness of a potential gene therapy treatment for patients with GSDIa. Under the agreement, the Company is obligated to make noncancelable research funding payments of $244 annually, aggregating total payments of $732. Legal Proceedings The Company is not currently party to any material legal proceedings. At each reporting date, the Company evaluates whether or not a potential loss amount or a potential range of loss is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. The Company expenses as incurred the costs related to its legal proceedings. Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company is not aware of any claims under indemnification arrangements, and it has not accrued any liabilities related to such obligations in its condensed financial statements as of September 30, 2015.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2015
Subsequent Events [Abstract]
Subsequent Events	12. Subsequent Events Reverse Stock Split On October 9, 2015, the Company effected a one-for-2.923 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company’s Convertible Preferred Stock. Accordingly, all share and per share amounts for all periods presented in the accompanying condensed financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split and adjustment of the preferred stock conversion ratios. Initial Public Offering On October 27, 2015, the Company completed an IPO, which resulted in the issuance and sale of 5,500,000 shares of its common stock at a public offering price of $13.00 per share, resulting in net proceeds of approximately $63,445 after deducting underwriting discounts and other estimated offering costs. Upon the closing of the IPO, all outstanding shares of the Company’s Convertible Preferred Stock were automatically converted into 15,286,968 shares of the Company’s common stock. On November 25, 2015, the underwriters of the Company’s IPO exercised their over-allotment option to purchase an additional 104,775 shares of common stock at the initial public offering price of $13.00 per share, resulting in net proceeds of approximately $1,250, after deducting underwriting discounts and other estimated offering costs. Operating Lease On November 2, 2015, the Company entered into an operating lease agreement with ARE-MA Region No. 20, LLC for laboratory and office space located at 19 Presidential Way, Woburn, Massachusetts (see Note 11).

Summary of Significant Accoun18

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2015
Accounting Policies [Abstract]
Unaudited Interim Financial Information	Unaudited Interim Financial Information The accompanying condensed balance sheet as of September 30, 2015, the condensed statements of operations and comprehensive loss for the three and nine months ended September 30, 2015 and 2014 and of condensed cash flows for the nine months ended September 30, 2015 and 2014 are unaudited. The unaudited interim condensed financial statements have been prepared on the same basis as the audited annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of September 30, 2015 and the results of its operations and its cash flows for the nine months ended September 30, 2015 and 2014. The financial data and other information disclosed in the notes related to the three and nine months ended September 30, 2015 and 2014 are unaudited. The results for the nine months ended September 30, 2015 are not necessarily indicative of results to be expected for the year ending December 31, 2015, any other interim periods, or any future year or period. The unaudited interim financial statements of the Company included herein have been prepared pursuant to the rules and regulations of the SEC. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted from this report, as is permitted by such rules and regulations. These unaudited condensed financial statements should be read in conjunction with the audited financial statements and the notes thereto for the year ended December 31, 2014 included in the Company’s final prospectus dated October 21, 2015, filed pursuant to Rule 424(b)(4) under the Securities Act of 1933 on October 22, 2015.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date In August 2014, the FASB issued ASU No. 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern In April 2015, the FASB issued ASU No. 2015-03, Interest — Imputation of Interest Subtopic 835-30 Income Taxes (Subt opic 740)

Fair Value of Financial Asset19

Fair Value of Financial Assets and Liabilities (Tables)	9 Months Ended
	Sep. 30, 2015
Fair Value Disclosures [Abstract]
Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis	The following tables present information about the Company’s financial assets measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values: Fair Value Measurements as of September 30, 2015 using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ 69,245 $ — $ — $ 69,245 $ 69,245 $ — $ — $ 69,245 Fair Value Measurements as of December 31, 2014 using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ 17,836 $ — $ — $ 17,836 $ 17,836 $ — $ — $ 17,836

Accrued Expenses and Other Cu20

Accrued Expenses and Other Current Liabilities (Tables)	9 Months Ended
	Sep. 30, 2015
Payables and Accruals [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following: September 30, 2015 December 31, 2014 Payroll and payroll-related costs $ 1,307 $ 469 External research and development services 1,582 362 Professional fees 1,507 159 Other 53 90 $ 4,449 $ 1,080

Collaboration Agreement (Tables

Collaboration Agreement (Tables)	9 Months Ended
	Sep. 30, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Revenue from Collaboration Agreement	Revenue recognized under the collaboration agreement with Bayer consisted of the following: Three Months Ended September 30, Nine Months Ended September 30, 2015 2014 2015 2014 Revenue: Bayer Collaboration $ 2,061 $ 1,184 $ 5,397 $ 1,343 $ 2,061 $ 1,184 $ 5,397 $ 1,343

Related Party Transactions (Tab

Related Party Transactions (Tables)	9 Months Ended
	Sep. 30, 2015
Related Party Transactions [Abstract]
Summary of Related Party Expenses	Related party expenses consisted of the following: Three Months Ended September 30, Nine Months Ended September 30, 2015 2014 2015 2014 Related Parties: REGENX - R&D Expense $ 1,000 $ 1,445 $ 2,000 $ 4,881 University of Pennsylvania - R&D Expense 1,498 — 4,158 — Fidelity Biosciences - G&A Expense — 14 — 42 $ 2,498 $ 1,459 $ 6,158 $ 4,923

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of Stock Option Activity	The following table summarizes the Company’s stock option activity since December 31, 2014: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) Outstanding as of December 31, 2014 1,104,172 $ 0.53 9.7 $ 67 Granted 1,938,705 $ 4.06 Exercised (12,572 ) $ 0.38 Cancelled or forfeited (8,981 ) $ 0.38 Outstanding as of September 30, 2015 3,021,324 $ 2.80 9.4 $ 22,359 Options vested and expected to vest as of September 30, 2015 3,021,324 $ 2.80 9.4 $ 22,359 Options exercisable as of September 30, 2015 311,081 $ 1.24 9.0 $ 2,788
Summary of Restricted Common Stock Activity	The following table summarizes the Company’s restricted common stock activity: Number of Shares Weighted Average Grant Date Fair Value Unvested restricted common stock at December 31, 2014 210,287 $ 0.53 Issued 35,788 $ 3.57 Vested (72,067 ) $ 1.37 Unvested restricted common stock at September 30, 2015 174,008 $ 0.86
Summary of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock	Stock-based compensation expense related to stock options and restricted common stock was classified in the condensed statements of operations and comprehensive loss as follows: Three Months Ended September 30, Nine Months Ended September 30, 2015 2014 2015 2014 Research and development $ 318 $ 14 $ 487 $ 32 General and administrative 232 9 399 17 $ 550 $ 23 $ 886 $ 49

Net Loss per Share (Tables)

Net Loss per Share (Tables)	9 Months Ended
	Sep. 30, 2015
Earnings Per Share [Abstract]
Schedule of Basic and Diluted Net Loss per Share Attributable to Common Stockholders	Basic and diluted net loss per share attributable to common stockholders was calculated as follows: Three Months Ended September 30, Nine Months Ended September 30, 2015 2014 2015 2014 Numerator: Net loss $ (10,667 ) $ (2,484 ) $ (26,028 ) $ (8,350 ) Accretion of convertible preferred stock to redemption value — (18 ) (23 ) (52 ) Net loss attributable to common stockholders $ (10,667 ) $ (2,502 ) $ (26,051 ) $ (8,402 ) Denominator: Weighted average common shares outstanding 4,093,836 4,056,818 4,077,551 3,950,468 Less: Weighted average shares of unvested restricted common stock outstanding (186,640 ) (471,215 ) (196,772 ) (368,255 ) Weighted average common shares outstanding used in calculating net loss per share attributable to common stockholders 3,907,196 3,585,603 3,880,779 3,582,213 Net loss per share attributable to common stockholders $ (2.73 ) $ (0.70 ) $ (6.71 ) $ (2.35 )
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share	The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an antidilutive effect: Three and Nine Months Ended September 30, 2015 2014 Convertible preferred stock (as converted to common stock) 15,286,968 3,421,142 Options to purchase common stock 3,021,324 811,669 Unvested restricted common stock 174,008 469,211 Warrant for the purchase of common stock 11,973 11,973 18,494,273 4,713,995

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2015
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments under Operating Leases	The following table summarizes the future minimum lease payments under the Cambridge and Woburn operating leases: Year Ending December 31, 2015 (remainder of year) $ 173 2016 1,111 2017 1,203 2018 548 2019 503 2020 521 2021 131 Total $ 4,190

Nature of the Business and Ba26

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Nov. 25, 2015	Oct. 27, 2015	Sep. 30, 2015	Dec. 31, 2014
Nature Of Business And Basis Of Presentation [Line Items]
Accumulated deficit			$ 42,166	$ 16,138
Subsequent Event [Member] | Common Stock [Member] | IPO [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Number of shares issued		5,500,000
Shares issued, price per share		$ 13
Proceeds from issuance of initial public offering		$ 63,445
Number of common shares issued on conversion of convertible preferred stock		15,286,968
Subsequent Event [Member] | Common Stock [Member] | Overallotment Option Granted to Underwriters [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Number of shares issued	104,775
Shares issued, price per share	$ 13
Proceeds from issuance of initial public offering	$ 1,250

Fair Value of Financial Asset27

Fair Value of Financial Assets and Liabilities - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Assets:
Cash equivalents	$ 69,245	$ 17,836
Assets total	69,245	17,836
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents	69,245	17,836
Assets total	$ 69,245	$ 17,836

Accrued Expenses and Other Cu28

Accrued Expenses and Other Current Liabilities - Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Payables and Accruals [Abstract]
Payroll and payroll-related costs	$ 1,307	$ 469
External research and development services	1,582	362
Professional fees	1,507	159
Other	53	90
Total	$ 4,449	$ 1,080

Notes Payable - Additional Info

Notes Payable - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2015	Jan. 31, 2015
Line of Credit Facility [Line Items]
Principal repaid during period	$ 140	$ 248
Loan and Security Agreement [Member]
Line of Credit Facility [Line Items]
Accretion of debt discount	2	7
Unamortized debt discount	25	25
Debt instrument annual principal repayments remainder of fiscal year	146	146
Debt Instrument annual principal repayments year two	583	583
Debt Instrument annual principal repayments year three	583	583
Debt instrument annual principal repayments year four	$ 190	$ 190
Equipment Line of Credit [Member]
Line of Credit Facility [Line Items]
Borrowings under line of credit			$ 194

Collaboration Agreement - Addit

Collaboration Agreement - Additional Information (Detail) - Bayer Healthcare LLC [Member] - Collaboration Agreement [Member] - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront license payment received	$ 20,000,000
Research agreement period	54 months
Research agreement commencement date	2014-06
Research payments earned		$ 2,092,000	$ 1,290,000	$ 5,470,000	$ 1,290,000
Deferred revenue		$ 22,715,000		$ 22,715,000
Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential future milestone payments receivable under development and commercialization activities	$ 232,000,000

Collaboration Agreement - Sched

Collaboration Agreement - Schedule of Revenue from Collaboration Agreement (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Revenue:
Revenue	$ 2,061	$ 1,184	$ 5,397	$ 1,343
Collaboration Agreement [Member] | Bayer Healthcare LLC [Member]
Revenue:
Revenue	$ 2,061	$ 1,184	$ 5,397	$ 1,343

Related Party Transactions - Su

Related Party Transactions - Summary of Related Party Expenses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Related Parties:
Related party expenses	$ 2,498	$ 1,459	$ 6,158	$ 4,923
REGENXBIO, Inc [Member]
Related Parties:
Related party expenses	1,000	1,445	2,000	4,881
University of Pennsylvania [Member]
Related Parties:
Related party expenses	$ 1,498		$ 4,158
Fidelity Biosciences [Member]
Related Parties:
Related party expenses		$ 14		$ 42

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
REGENXBIO, Inc [Member]
Related Party Transaction [Line Items]
Cash payment to related party	$ 1,000,000	$ 2,083,000	$ 2,750,000	$ 5,896,000
Amounts due to related party	0		0		$ 750,000
University of Pennsylvania [Member]
Related Party Transaction [Line Items]
Cash payment to related party	1,000,000	0	4,137,000	0
Amounts due to related party	21,000		21,000		0
Fidelity Biosciences [Member]
Related Party Transaction [Line Items]
Cash payment to related party	0	$ 0	1,000	$ 38,000
Amounts due to related party	$ 0		$ 0		$ 0

Common and Convertible Prefer34

Common and Convertible Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Apr. 20, 2015	Feb. 02, 2015	Sep. 30, 2015	Sep. 30, 2014	Oct. 27, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Convertible preferred stock issued			44,683,824			10,000,000
Convertible preferred stock, par value			$ 0.0001			$ 0.0001
Proceeds from issuance of convertible preferred stock, gross			$ 79,219	$ 4,875
Stock issuance costs for convertible preferred stock			$ 1,501
Common stock, shares authorized			68,000,000			65,700,000
Common stock, par value			$ 0.0001			$ 0.0001
Undesignated preferred stock, shares authorized			44,683,824			45,000,000
Series A Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Convertible preferred stock issued		14,500,000
Shares issued, price per share		$ 1
Convertible preferred stock, par value		$ 0.0001
Proceeds from issuance of convertible preferred stock, gross		$ 14,500
Stock issuance costs for convertible preferred stock		$ 20
Series B Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Convertible preferred stock issued	20,183,824
Shares issued, price per share	$ 3.2204
Convertible preferred stock, par value	$ 0.0001
Proceeds from issuance of convertible preferred stock, gross	$ 65,000
Stock issuance costs for convertible preferred stock	$ 261
Subsequent Event [Member]
Class of Stock [Line Items]
Convertible preferred stock, par value					$ 0.0001
Common stock, shares authorized					150,000,000
Common stock, par value					$ 0.0001
Undesignated preferred stock, shares authorized					5,000,000

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Sep. 02, 2015	Sep. 30, 2015	Sep. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate			1.76%
Expected term			6 years
Expected volatility			72.98%
Dividend yield			0.00%
Weighted average grant-date fair value of stock options granted		$ 4.91	$ 4.06
Unvested service-based stock options held by nonemployees		3,934	3,934
Total unrecognized compensation cost related to the unvested awards		$ 5,108	$ 5,108
Unrecognized compensation cost related to the unvested awards, weighted-average period			3 years 6 months
Restricted Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total fair value of restricted common stock vested			$ 295
2013 Stock Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of estimated fair value of the shares on the date of grant			100.00%
Stock options expiration period			10 years
Common stock reserved for issuance		3,520,396	3,520,396
Common stock reserved for future grant		54,510	54,510
2013 Stock Plan [Member] | Employees And Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting period			4 years
2013 Stock Plan [Member] | Non Employees [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting period			2 years
2013 Stock Plan [Member] | Non Employees [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting period			4 years
2013 Stock Plan [Member] | Restricted Common Stock [Member] | Employees And Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted in connection with employee stock purchase plan			35,788
2013 Stock Plan [Member] | Options to Purchase Common Stock [Member] | Employees And Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted in connection with employee stock purchase plan			1,938,705
2015 Stock Option and Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance	1,422,886
Common stock reserved for issuance, percentage of number of shares of common stock outstanding	4.00%
2015 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance	256,585
Common stock reserved for issuance, percentage of number of shares of common stock outstanding	1.00%
2015 Employee Stock Purchase Plan [Member] | Non Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted in connection with employee stock purchase plan			0

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Options Outstanding, Weighted Average Exercise Price, and Additional Disclosures [Abstract]
Outstanding, Number of Shares, Beginning Balance	1,104,172
Granted, Number of Shares	1,938,705
Exercised, Number of Shares	(12,572)
Cancelled or forfeited, Number of Shares	(8,981)
Outstanding, Number of Shares, Ending Balance	3,021,324	1,104,172
Options vested and expected to vest, Number of Shares	3,021,324
Options exercisable, Number of Shares	311,081
Outstanding, Weighted Average Exercise Price, Beginning Balance	$ 0.53
Granted, Weighted Average Exercise Price	4.06
Exercised, Weighted Average Exercise Price	0.38
Cancelled or forfeited, Weighted Average Exercise Price	0.38
Outstanding, Weighted Average Exercise Price, Ending Balance	2.80	$ 0.53
Options vested and expected to vest, Weighted Average Exercise Price	2.80
Options exercisable, Weighted Average Exercise Price	$ 1.24
Outstanding, Weighted Average Remaining Contractual Term	9 years 4 months 24 days	9 years 8 months 12 days
Options vested and expected to vest, Aggregate Intrinsic Value	9 years 4 months 24 days
Options exercisable, Weighted Average Remaining Contractual Term	9 years
Outstanding, Aggregate Intrinsic Value, Beginning Balance	$ 67
Outstanding, Aggregate Intrinsic Value, Ending Balance	22,359	$ 67
Options vested and expected to vest, Number of Shares	22,359
Options exercisable, Aggregate Intrinsic Value	$ 2,788

Stock-Based Compensation - Su37

Stock-Based Compensation - Summary of Restricted Common Stock Activity (Detail) - Restricted Common Stock [Member]	9 Months Ended
	Sep. 30, 2015$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested restricted common stock, Number of Shares, Beginning Balance | shares	210,287
Issued, Number of Shares | shares	35,788
Vested, Number of Shares | shares	(72,067)
Unvested restricted common stock, Number of Shares, Ending Balance | shares	174,008
Unvested restricted common stock, Weighted Average Grant Date Fair Value, Beginning Balance	$ 0.53
Issued, Weighted Average Grant Date Fair Value	3.57
Vested, Weighted Average Grant Date Fair Value	1.37
Unvested restricted common stock, Weighted Average Grant Date Fair Value, Ending Balance	$ 0.86

Stock-Based Compensation - Su38

Stock-Based Compensation - Summary of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 550	$ 23	$ 886	$ 49
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	318	14	487	32
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 232	$ 9	$ 399	$ 17

Net Loss per Share - Schedule o

Net Loss per Share - Schedule of Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Numerator:
Net loss	$ (10,667)	$ (2,484)	$ (26,028)	$ (8,350)
Accretion of convertible preferred stock to redemption value		(18)	(23)	(52)
Net loss attributable to common stockholders	$ (10,667)	$ (2,502)	$ (26,051)	$ (8,402)
Denominator:
Weighted average common shares outstanding - basic and diluted	4,093,836	4,056,818	4,077,551	3,950,468
Less: Weighted average shares of unvested restricted common stock outstanding	(186,640)	(471,215)	(196,772)	(368,255)
Weighted average common shares outstanding used in calculating net loss per share attributable to common stockholders - basic and diluted	3,907,196	3,585,603	3,880,779	3,582,213
Net loss per share attributable to common stockholders - basic and diluted	$ (2.73)	$ (0.70)	$ (6.71)	$ (2.35)

Net Loss per Share - Schedule40

Net Loss per Share - Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share (Detail) - shares	3 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	18,494,273	4,713,995
Convertible Preferred Stock (as Converted to Common Stock) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	15,286,968	3,421,142
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	3,021,324	811,669
Restricted Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	174,008	469,211
Warrant for the Purchase of Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	11,973	11,973

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	Nov. 02, 2015USD ($)ft²$ / ft²	Jun. 30, 2015USD ($)	Sep. 30, 2015USD ($)	Sep. 30, 2014USD ($)	Sep. 30, 2015USD ($)	Sep. 30, 2014USD ($)
Commitments and Contingencies [Line Items]
Operating leases, rent expense			$ 171,000	$ 71,000	$ 563,000	$ 157,000
ARE-MA Region No.20 LLC [Member] | Laboratory and Office Facility [Member] | Subsequent Event [Member]
Commitments and Contingencies [Line Items]
Operating lease, area of property leased | ft²	17,500
Operating lease, term	5 years
Operating lease, annual rent	$ 400,000
Operating lease, annual increase in rent per square foot | $ / ft²	1
Operating lease, tenant improvement allowance per rentable square foot | $ / ft²	35
Eunice Kennedy Shriver National Institute of Child Health and Human Development [Member]
Commitments and Contingencies [Line Items]
Research and development agreement period		3 years
Noncancelable research funding payments minimum amount committed		$ 244,000
Research funding potential aggregate amount		$ 732,000

Commitments and Contingencies42

Commitments and Contingencies - Schedule of Future Minimum Lease Payments under Operating Leases (Detail) - Cambridge and Woburn Operating Leases [Member] $ in Thousands	Sep. 30, 2015USD ($)
Operating Leased Assets [Line Items]
2015 (remainder of year)	$ 173
2,016	1,111
2,017	1,203
2,018	548
2,019	503
2,020	521
2,021	131
Total	$ 4,190

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - Subsequent Event [Member] $ / shares in Units, $ in Thousands	Nov. 25, 2015USD ($)$ / sharesshares	Oct. 27, 2015USD ($)$ / sharesshares	Oct. 09, 2015
Subsequent Event [Line Items]
Reverse stock split conversion description			The Company effected a one-for-2.923 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company's Convertible Preferred Stock.
Reverse stock split conversion ratio			0.34211
Common Stock [Member] | IPO [Member]
Subsequent Event [Line Items]
Number of shares issued		5,500,000
Shares issued, price per share | $ / shares		$ 13
Proceeds from issuance of initial public offering | $		$ 63,445
Number of common shares issued on conversion of convertible preferred stock		15,286,968
Common Stock [Member] | Overallotment Option Granted to Underwriters [Member]
Subsequent Event [Line Items]
Number of shares issued	104,775
Shares issued, price per share | $ / shares	$ 13
Proceeds from issuance of initial public offering | $	$ 1,250