Dimension Therapeutics (DMTX) 10-Q 10 May 17 2017 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2017	May 03, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q1
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	Accelerated Filer
Entity Common Stock, Shares Outstanding		25,043,506

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 24,037	$ 30,234
Marketable securities	35,107	47,715
Accounts receivable	2,365	1,885
Prepaid expenses and other current assets	8,630	5,484
Total current assets	70,139	85,318
Property and equipment, net	7,883	8,402
Deferred offering costs	205	145
Total assets	78,227	93,865
Current liabilities:
Accounts payable	1,411	2,368
Accrued expenses and other current liabilities	4,298	7,247
Deferred revenue	9,184	8,663
Notes payable	2,500	2,361
Total current liabilities	17,393	20,639
Deferred revenue, net of current portion	9,339	8,663
Notes payable, net of discount and current portion	3,584	4,169
Other liabilities	422	453
Total liabilities	30,738	33,924
Commitments and contingencies (Note 9)
Stockholders’ equity:
Preferred stock, $0.0001 par value; 5,000,000 shares authorized at March 31, 2017 and December 31, 2016; zero shares issued or outstanding at March 31, 2017 and December 31, 2016.	0	0
Common stock, $0.0001 par value; 150,000,000 shares authorized as of March 31, 2017 and December 31, 2016; 25,043,506 shares issued and outstanding as of March 31, 2017 and December 31, 2016.	2	2
Additional paid-in capital	161,225	160,185
Accumulated deficit	(113,687)	(100,195)
Accumulated other comprehensive loss	(51)	(51)
Total stockholders’ equity	47,489	59,941
Total liabilities and stockholders’ equity	$ 78,227	$ 93,865

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Mar. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
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	$ 0.0001	$ 0.0001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	25,043,506	25,043,506
Common stock, shares outstanding	25,043,506	25,043,506

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Revenue	$ 3,618	$ 2,206
Operating expenses:
Research and development	13,714	8,805
General and administrative	3,432	2,941
Total operating expenses	17,146	11,746
Loss from operations	(13,528)	(9,540)
Interest income, net	36	28
Net loss	$ (13,492)	$ (9,512)
Net loss per share — basic and diluted	$ (0.54)	$ (0.38)
Weighted average common shares outstanding — basic and diluted	24,981,678	24,851,933
Comprehensive loss:
Net loss	$ (13,492)	$ (9,512)
Other comprehensive loss:
Unrealized loss on marketable securities	(51)
Total other comprehensive loss	(51)
Total comprehensive loss	$ (13,543)	$ (9,512)

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net loss	$ (13,492)	$ (9,512)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	543	216
Stock-based compensation expense	1,040	533
Non-cash interest expense	45	2
Amortization of premium on marketable securities	8	0
Changes in operating assets and liabilities:
Accounts receivable	(480)	(1,121)
Prepaid expenses and other current assets	(3,146)	96
Accounts payable	(957)	424
Accrued expenses and other current liabilities	(2,949)	(1,040)
Deferred revenue	1,197	1,074
Other liabilities	(31)	(10)
Net cash used in operating activities	(18,222)	(9,338)
Cash flows from investing activities:
Sales and maturities of marketable securities	12,600	0
Purchases of property and equipment	(24)	(544)
Net cash provided (used) in investing activities	12,576	(544)
Cash flows from financing activities:
Proceeds from exercise of common stock options	0	7
Repayment of notes payable	(491)	(145)
Payments of offering costs	(60)	0
Net cash used by financing activities	(551)	(138)
Net decrease in cash and cash equivalents	(6,197)	(10,020)
Cash and cash equivalents at beginning of period	30,234	127,047
Cash and cash equivalents at end of period	24,037	117,027
Supplemental disclosure of cash flow information:
Interest paid	52	26
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable, accrued expenses and other liabilities	20	2,578
Public offering costs included in accounts payable and accrued expenses	$ 10	$ 0

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	3 Months Ended
	Mar. 31, 2017
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 leader in discovering and developing new therapeutic products for people living with devastating rare and metabolic diseases associated with the liver, based on the most advanced, mammalian adeno-associated virus (AAV) gene delivery technology. The liver is a vital organ that plays an important role in human metabolism and key physiologic functions, and is the target organ for our initial programs: OTC deficiency, GSDIa, phenylketonuria, Wilson disease, citrullinemia type I, and hemophilia A. 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. In accordance with ASC 205-40, Going Concern, the Company has evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the condensed consolidated financial statements are issued. As of March 31, 2017, the Company had an accumulated deficit of $113,687. During the three months ended March 31, 2017, the Company incurred a loss of $13,492 and used $18,222 of cash in operations. The Company expects to continue to generate operating losses in the foreseeable future. The Company expects that its cash, cash equivalents and marketable securities of $59,144 at March 31, 2017, together with the receipt of contingent payments expected to be received in connection with our collaboration with Bayer, including reimbursements and $15,000 in milestones, and cost management initiatives, will be sufficient to fund its operating expenses and capital expenditure requirements for at least the next 12 months from issuance of the financial statements. If the Company does not obtain the milestone payments, the Company would be forced to delay, reduce or eliminate certain research and development programs, reduce or eliminate discretionary operating expenses, delay product portfolio expansion, which could adversely affect its business prospects. In addition, the Company will seek additional funding through public or private financings, debt financing, collaboration agreements or government grants. The inability to obtain funding, as and when needed, would have a negative impact on the Company’s financial condition and ability to pursue its business strategies. The condensed consolidated financial statements 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	3 Months Ended
	Mar. 31, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation and Principles of Consolidation The accompanying condensed consolidated financial statements reflect the operations of Dimension Therapeutics, Inc. and Dimension Securities Corporation, our wholly-owned subsidiary. All intercompany accounts and transactions have been eliminated. The condensed consolidated financial statements are unaudited and have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”). The accounting policies followed in the preparation of the interim condensed consolidated financial statements are consistent in all material respects with those presented in Note 2 to the financial statements included on the Annual Report on Form 10-K for the fiscal year ended on December 31, 2016. Unaudited Interim Financial Information The condensed consolidated balance sheets at December 31, 2016 was derived from audited financial statements, but does not include all disclosures required by GAAP. The accompanying condensed consolidated balance sheets as of March 31, 2017, the condensed consolidated statements of operations and comprehensive loss for the three months ended March 31, 2017 and 2016 and the condensed consolidated statement of cash flows for the three months ended March 31, 2017 and 2016 are unaudited. The unaudited interim condensed consolidated 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 March 31, 2017, the results of its operations for the three months ended March 31, 2017 and 2016, and its statement of cash flows for the three months ended March 31, 2017 and 2016. The financial data and other information disclosed in the notes related to the three months ended March 31, 2017 and 2016 are unaudited. The results for the three months ended March 31, 2017 and 2016 are not necessarily indicative of results to be expected for the year ending December 31, 2017, 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 consolidated financial statements should be read in conjunction with the audited financial statements and the notes thereto for the year ended December 31, 2016 included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2016. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates, assumptions and judgments that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements, and the reported amounts of revenue and expenses during the reporting periods. Significant estimates, assumptions and judgments reflected in these condensed consolidated financial statements include, but are not limited to, the accrual of research and development expenses, the recognition of revenue from the Company’s collaboration with Bayer, the valuation of common stock prior to the IPO and the valuation of stock-based awards. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates. Recently Issued Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2014-09, Revenue from Contracts with Customers (Topic 606) Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients, In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) In March 2016, the FASB issued ASU 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting In March 2017 the FASB issued ASU No. 2017-08, Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities.

Fair Value Measurements and Mar

Fair Value Measurements and Marketable Securities	3 Months Ended
	Mar. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value Measurements and Marketable Securities	3. Fair Value Measurements and Marketable Securities Fair Value Measurements The Company’s cash equivalents are generally classified within Level 1 of the fair value hierarchy. The Company’s investments in marketable securities are classified within Level 2 of the fair value hierarchy. The fair values of the Company’s marketable securities are generally based on prices obtained from independent pricing sources. Consistent with the fair value hierarchy described above, securities with validated quotes from pricing services are generally reflected within Level 2, as they are primarily based on observable pricing for similar assets and/or other market observable inputs. Typical inputs used by these pricing services include, but are not limited to, reported trades, benchmark yields, issuer spreads, bids, offers and/or estimates cash flow, prepayment spreads and default rates. 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 March 31, 2017 using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 24,037 $ — $ — $ 24,037 Total cash equivalents 24,037 — — 24,037 Marketable securities: Commercial paper — 6,744 — 6,744 Corporate bonds — 13,641 — 13,641 U.S. government securities — 14,722 — 14,722 Total marketable securities — 35,107 — 35,107 Total cash equivalents and marketable securities $ 24,037 $ 35,107 $ — $ 59,144 Fair Value Measurements as of December 31, 2016 Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 22,988 $ — $ — $ 22,988 U.S. government securities — 7,246 — 7,246 Total cash equivalents 22,988 7,246 — 30,234 Marketable securities: Commercial paper — 15,814 — 15,814 Corporate Bonds — 13,671 — 13,671 U.S. government securities — 18,230 — 18,230 Total marketable securities — 47,715 — 47,715 Total cash equivalents and marketable securities $ 22,988 $ 54,961 $ — $ 77,949 During the three months ended March 31, 2017 and the year ended December 31, 2016, there were no transfers between Level 1 and Level 2. The carrying amounts reflected in the condensed consolidated balance sheets for accounts payable and accrued expenses approximate fair value due to their short-term maturities. The carrying value of the Company’s outstanding notes payable approximates fair value (a Level 2 fair value measurement), reflecting interest rates currently available to the Company. Marketable Securities The following tables summarize the Company’s marketable securities as of March 31, 2017 and December 31, 2016: Fair Value Measurements as of March 31, 2017 using: Amortized Cost Gross Unrealized Gains Gross Unrealized Loss Fair Value Assets: Commercial paper (due within 1 year) $ 6,744 $ — $ — $ 6,744 Corporate Bonds (due within 1 year) $ 13,657 $ — $ (16 ) $ 13,641 U.S. government securities (due within 1 year) 14,755 — (33 ) 14,722 $ 35,156 $ — $ (49 ) $ 35,107 Fair Value Measurements as of December 31, 2016 using: Amortized Cost Gross Unrealized Gains Gross Unrealized Loss Fair Value Assets: Commercial paper (due within 1 year) $ 15,814 $ — $ — $ 15,814 Corporate Bonds (due within 1 year) $ 9,182 $ — $ (10 ) $ 9,172 U.S. government securities (due within 1 year) 7,002 — — 7,002 Corporate Bonds (due after 1 year through 2 years) 4,512 — (12 ) 4,500 U.S. government securities (due after 1 year through 2 years) 11,256 — (29 ) 11,227 $ 47,766 $ — $ (51 ) $ 47,715

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	3 Months Ended
	Mar. 31, 2017
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: March 31, December 31, 2017 2016 External research and development services $ 2,742 $ 4,409 Payroll and payroll related costs 759 2,234 Professional services 571 433 Other 226 171 $ 4,298 $ 7,247

Notes Payable

Notes Payable	3 Months Ended
	Mar. 31, 2017
Debt Disclosure [Abstract]
Notes Payable	5. Notes Payable Through March 31, 2017, we had borrowed a total of $7,551 available under the loan and security agreement, as amended. During the three months ended March 31, 2017, we made $491 of scheduled principal repayments. In addition, the Company accrues the related total final payments due of $348 to outstanding debt by charges to interest expense using the effective-interest method from the date of issuance through the maturity date. As of March 31, 2017 and December 31, 2016, notes payable consist of the following: March 31, December 31, 2017 2016 Notes payable $ 5,987 $ 6,478 Less: current portion (2,500 ) (2,361 ) Notes payable, net of current portion 3,487 4,117 Debt discount, net of accretion (25 ) (30 ) Accretion related to final payment 122 82 Notes payable, net of discount, long term $ 3,584 $ 4,169 Estimated future principal payments due under the loan and security agreement with Silicon Valley Bank (“SVB”) as of March 31, 2017 are as follows: 2017 (remainder of year) $ 1,888 2018 2,123 2019 1,838 2020 138 Total $ 5,987 During the three months ended March 31, 2017 and 2016, the Company recognized $115 and $20, respectively, of interest expense related to the loan and security agreement with SVB.

Collaboration and License Agree

Collaboration and License Agreements	3 Months Ended
	Mar. 31, 2017
Collaboration And License Agreements [Abstract]
Collaboration and License Agreements	6. Collaboration and License Agreements Bayer HealthCare LLC In June 2014, we entered into an agreement with Bayer to research, develop and commercialize AAV gene therapy products for treatment of hemophilia A. The research term of the agreement is 54 months. Under this agreement, Bayer has been granted an exclusive license to develop and commercialize one or more novel gene therapies for hemophilia A. We are responsible for the development of DTX201 under the agreement through a proof-of-concept clinical trial, with full reimbursement from Bayer for all project costs in accordance with the mutually agreed upon research budget. Bayer is responsible operationally and will incur the costs of the conduct of the proof-of-concept clinical trial. Upon the successful demonstration of clinical proof of concept, the agreement requires that Bayer use commercially reasonable efforts to manage and fund any subsequent clinical trials and commercialization of gene therapy products for treatment of hemophilia A. 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 March 31, 2017, 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 March 31, 2017 2016 Revenue: Bayer Collaboration $ 3,618 $ 2,206 $ 3,618 $ 2,206 During the three months ended March 31, 2017 and 2016, research payments earned by the Company under the collaboration agreement totaled $2,365 and $1,264, respectively. As of March 31, 2017, the unbilled amount of earned research payments was $2,365. 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 consolidated statements of operations and comprehensive loss. As of March 31, 2017 and December 31, 2016, deferred revenue related to the collaboration agreement with Bayer totaled $18,524 and $17,326, respectively, which related to the upfront license payment, research services payments and 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.

Stock-Based Compensation

Stock-Based Compensation	3 Months Ended
	Mar. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	7. Stock-Based Compensation 2015 Stock Option and Incentive Plan In September 2015, the Company’s stockholders approved the 2015 Stock Option and Incentive Plan (the “2015 Plan”). 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. The 2015 Stock Option Plan replaced the Company’s 2013 Stock Option and Grant Plan (the “2013 Plan”). The Company will grant no future stock options or other awards under the 2013 Stock Option Plan. Any options or awards outstanding under the 2013 Stock Option Plan remained outstanding and effective. As of March 31, 2017, the total number of shares reserved under the 2015 Plan and the 2013 Stock Option Plan was 6,890,841 and the Company had 1,270,764 shares available for future issuance under the 2015 Stock Option Plan. The 2015 Plan provides for an annual increase, to be added 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. On January 1, 2017, 1,001,740 shares of common stock, representing 4% of the Company’s issued and outstanding shares of common stock as of December 31, 2016, were added to the 2015 Plan. Such shares are included in the equity plan totals specified in the paragraph above. During the three months ended March 31, 2017, the Company granted to employees and directors no shares of restricted common stock and options to purchase 1,008,300 shares of common stock. During the three months ended March 31, 2017, the Company did not grant to non-employees any stock-based awards. 2015 Employee Stock Purchase Plan As of March 31, 2017, the total number of shares reserved and available under the 2015 Employee Stock Purchase Plan (“ESPP”) was 507,020. As of March 31, 2017, there was no activity under the ESPP. The ESPP provides for an annual increase, to be added on the first day of each fiscal year starting on January 1 2017 and ending on January 2021, by the lesser of 750,000 shares of common stock, up to 1% of the Company’s issued and outstanding shares of common stock on the immediately preceding December 31, or such number of shares of common stock as determined by the Company’s board of directors or the compensation committee of the board of directors. On January 1, 2017, 250,435 shares of common stock, representing 1% of the Company’s issued and outstanding shares of common stock as of December 31, 2016, were added to the ESPP. Such shares are included in the ESPP totals specified in the paragraph above. Stock Options The following table summarizes the Company’s stock option activity since December 31, 2016: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) Outstanding as of December 31, 2016 4,185,699 $ 4.26 8.4 $ 4,757 Granted 1,008,300 $ 2.00 Exercised — — — Cancelled (53,763 ) $ 9.88 Outstanding as of March 31, 2017 5,140,236 $ 3.75 8.5 $ 1,281 Options vested and expected to vest as of March 31, 2017 5,140,236 $ 3.75 8.5 $ 1,281 Options exercisable as of March 31, 2017 1,803,796 $ 3.21 8.0 $ 818 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 March 31, 2017. The weighted average grant-date fair value of stock options granted during the three months ended March 31, 2017, was $2.00 per share. 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, 2016 70,053 $ 0.88 Issued — Vested (20,793 ) $ 0.85 Unvested restricted common stock at March 31, 2017 49,260 $ 0.89 The total fair value of restricted common stock vested during the three months ended March 31, 2017 was $36. Stock-Based Compensation Stock-based compensation expense related to stock options and restricted common stock was classified in the condensed consolidated statements of operations and comprehensive loss as follows: Three Months Ended March 31, 2017 2016 Research and development $ 380 $ 217 General and administrative 660 316 $ 1,040 $ 533 As of March 31, 2017, total unrecognized compensation cost related to the unvested awards was $8,044, which is expected to be recognized over a weighted average period of 2.54 years.

Net Loss per Share

Net Loss per Share	3 Months Ended
	Mar. 31, 2017
Earnings Per Share [Abstract]
Net Loss per Share	8 . Net Loss per Share Basic and diluted net loss per share attributable to common stockholders was calculated as follows: Three Months Ended March 31, 2017 2016 Numerator: Net loss $ (13,492 ) $ (9,512 ) Denominator: Weighted average common shares outstanding — basic and diluted 25,043,506 24,996,921 Less: Weighted average shares of unvested restricted common stock outstanding (61,828 ) (144,988 ) Weighted average common shares outstanding used in calculating net loss per share attributable to common stockholders — basic and diluted 24,981,678 24,851,933 Net loss per share attributable to common stockholders — basic and diluted $ (0.54 ) $ (0.38 ) The Company’s potential dilutive securities, which include stock options and unvested restricted 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 are 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 Months Ended March 31, 2017 2016 Options to purchase common stock 5,140,236 3,842,903 Unvested restricted common stock 49,260 132,428 5,189,496 3,975,331

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	9. Commitments and Contingencies Leases The Company leases office and laboratory space under an operating lease agreement, as amended, for its Cambridge, MA location with a term expiring January 2018 and an option to extend the lease for two additional years. The Company also operating lease agreement for its Woburn, MA location with a term expiring March 2021 and an option to extend the lease for five additional years . During the three months ended March 31, 2017 and 2016, the Company recognized $295 and $171 of rental expense related to office and laboratory space, respectively. The following table summarizes the future minimum lease payments under the Cambridge and Woburn operating leases as of March 31, 2017: Year Ending December 31, 2017 (remainder of year) $ 910 2018 550 2019 505 2020 523 2021 132 Total $ 2,620 The Company is further responsible for its pro rata share of operating expenses, real estate taxes and utilities of the facilities housing the leased office and laboratory space. 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 and certain executive officers 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 consolidated financial statements as of March 31, 2017.

Subsequent Events

Subsequent Events	3 Months Ended
	Mar. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	10. Subsequent Events Notes Payable On April 18, 2017, the Company borrowed $276 under the Second Amendment to the loan and security agreement with SVB. No additional issuance costs were accrued. In accordance with the terms of the Second Amendment, the borrowed loan amount accrues interest at an annual rate of 3.5%. Under the loan and security agreement we are obligated to make six monthly, interest-only payments related to the borrowed amount. Thereafter, we are obligated to pay 36 monthly installments of interest and principal. In addition, a final payment equal to 6% of the original principal amount is due on the earlier of the maturity date of the advance, acceleration of the terms of the loan, prepayment of the advance or termination of the loan and security agreement. Operating Lease On May 2, 2017, the Company entered into a second amendment to the lease agreement with Rivertech Associates II, LLC for its existing laboratory and office space located at 840 Memorial Drive, Cambridge, Massachusetts. That lease was to expire in January 2018. As amended, the lease will continue for a term of two years through January 2020 with an option to further extend the lease for five additional years. As amended, required lease payments include base rent of $80 per month through January 2019, which increases to $82 per month over the course of the amended lease through January 2020.

Summary of Significant Accoun16

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	Basis of Presentation and Principles of Consolidation The accompanying condensed consolidated financial statements reflect the operations of Dimension Therapeutics, Inc. and Dimension Securities Corporation, our wholly-owned subsidiary. All intercompany accounts and transactions have been eliminated. The condensed consolidated financial statements are unaudited and have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”). The accounting policies followed in the preparation of the interim condensed consolidated financial statements are consistent in all material respects with those presented in Note 2 to the financial statements included on the Annual Report on Form 10-K for the fiscal year ended on December 31, 2016.
Unaudited Interim Financial Information	Unaudited Interim Financial Information The condensed consolidated balance sheets at December 31, 2016 was derived from audited financial statements, but does not include all disclosures required by GAAP. The accompanying condensed consolidated balance sheets as of March 31, 2017, the condensed consolidated statements of operations and comprehensive loss for the three months ended March 31, 2017 and 2016 and the condensed consolidated statement of cash flows for the three months ended March 31, 2017 and 2016 are unaudited. The unaudited interim condensed consolidated 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 March 31, 2017, the results of its operations for the three months ended March 31, 2017 and 2016, and its statement of cash flows for the three months ended March 31, 2017 and 2016. The financial data and other information disclosed in the notes related to the three months ended March 31, 2017 and 2016 are unaudited. The results for the three months ended March 31, 2017 and 2016 are not necessarily indicative of results to be expected for the year ending December 31, 2017, 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 consolidated financial statements should be read in conjunction with the audited financial statements and the notes thereto for the year ended December 31, 2016 included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2016.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates, assumptions and judgments that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements, and the reported amounts of revenue and expenses during the reporting periods. Significant estimates, assumptions and judgments reflected in these condensed consolidated financial statements include, but are not limited to, the accrual of research and development expenses, the recognition of revenue from the Company’s collaboration with Bayer, the valuation of common stock prior to the IPO and the valuation of stock-based awards. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2014-09, Revenue from Contracts with Customers (Topic 606) Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients, In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) In March 2016, the FASB issued ASU 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting In March 2017 the FASB issued ASU No. 2017-08, Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities.

Fair Value Measurements and M17

Fair Value Measurements and Marketable Securities (Tables)	3 Months Ended
	Mar. 31, 2017
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 March 31, 2017 using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 24,037 $ — $ — $ 24,037 Total cash equivalents 24,037 — — 24,037 Marketable securities: Commercial paper — 6,744 — 6,744 Corporate bonds — 13,641 — 13,641 U.S. government securities — 14,722 — 14,722 Total marketable securities — 35,107 — 35,107 Total cash equivalents and marketable securities $ 24,037 $ 35,107 $ — $ 59,144 Fair Value Measurements as of December 31, 2016 Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 22,988 $ — $ — $ 22,988 U.S. government securities — 7,246 — 7,246 Total cash equivalents 22,988 7,246 — 30,234 Marketable securities: Commercial paper — 15,814 — 15,814 Corporate Bonds — 13,671 — 13,671 U.S. government securities — 18,230 — 18,230 Total marketable securities — 47,715 — 47,715 Total cash equivalents and marketable securities $ 22,988 $ 54,961 $ — $ 77,949
Summary of Marketable Securities	The following tables summarize the Company’s marketable securities as of March 31, 2017 and December 31, 2016: Fair Value Measurements as of March 31, 2017 using: Amortized Cost Gross Unrealized Gains Gross Unrealized Loss Fair Value Assets: Commercial paper (due within 1 year) $ 6,744 $ — $ — $ 6,744 Corporate Bonds (due within 1 year) $ 13,657 $ — $ (16 ) $ 13,641 U.S. government securities (due within 1 year) 14,755 — (33 ) 14,722 $ 35,156 $ — $ (49 ) $ 35,107 Fair Value Measurements as of December 31, 2016 using: Amortized Cost Gross Unrealized Gains Gross Unrealized Loss Fair Value Assets: Commercial paper (due within 1 year) $ 15,814 $ — $ — $ 15,814 Corporate Bonds (due within 1 year) $ 9,182 $ — $ (10 ) $ 9,172 U.S. government securities (due within 1 year) 7,002 — — 7,002 Corporate Bonds (due after 1 year through 2 years) 4,512 — (12 ) 4,500 U.S. government securities (due after 1 year through 2 years) 11,256 — (29 ) 11,227 $ 47,766 $ — $ (51 ) $ 47,715

Accrued Expenses and Other Cu18

Accrued Expenses and Other Current Liabilities (Tables)	3 Months Ended
	Mar. 31, 2017
Payables And Accruals [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following: March 31, December 31, 2017 2016 External research and development services $ 2,742 $ 4,409 Payroll and payroll related costs 759 2,234 Professional services 571 433 Other 226 171 $ 4,298 $ 7,247

Notes Payable (Tables)

Notes Payable (Tables)	3 Months Ended
	Mar. 31, 2017
Debt Disclosure [Abstract]
Summary of Notes Payable	As of March 31, 2017 and December 31, 2016, notes payable consist of the following: March 31, December 31, 2017 2016 Notes payable $ 5,987 $ 6,478 Less: current portion (2,500 ) (2,361 ) Notes payable, net of current portion 3,487 4,117 Debt discount, net of accretion (25 ) (30 ) Accretion related to final payment 122 82 Notes payable, net of discount, long term $ 3,584 $ 4,169
Estimated Future Principal Payments Due	Estimated future principal payments due under the loan and security agreement with Silicon Valley Bank (“SVB”) as of March 31, 2017 are as follows: 2017 (remainder of year) $ 1,888 2018 2,123 2019 1,838 2020 138 Total $ 5,987

Collaboration and License Agr20

Collaboration and License Agreements (Tables)	3 Months Ended
	Mar. 31, 2017
Collaboration And License Agreements [Abstract]
Schedule of Revenue from Collaboration Agreement	Revenue recognized under the collaboration agreement with Bayer consisted of the following: Three Months Ended March 31, 2017 2016 Revenue: Bayer Collaboration $ 3,618 $ 2,206 $ 3,618 $ 2,206

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock Option Activity	The following table summarizes the Company’s stock option activity since December 31, 2016: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) Outstanding as of December 31, 2016 4,185,699 $ 4.26 8.4 $ 4,757 Granted 1,008,300 $ 2.00 Exercised — — — Cancelled (53,763 ) $ 9.88 Outstanding as of March 31, 2017 5,140,236 $ 3.75 8.5 $ 1,281 Options vested and expected to vest as of March 31, 2017 5,140,236 $ 3.75 8.5 $ 1,281 Options exercisable as of March 31, 2017 1,803,796 $ 3.21 8.0 $ 818
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, 2016 70,053 $ 0.88 Issued — Vested (20,793 ) $ 0.85 Unvested restricted common stock at March 31, 2017 49,260 $ 0.89
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 consolidated statements of operations and comprehensive loss as follows: Three Months Ended March 31, 2017 2016 Research and development $ 380 $ 217 General and administrative 660 316 $ 1,040 $ 533

Net Loss per Share (Tables)

Net Loss per Share (Tables)	3 Months Ended
	Mar. 31, 2017
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 March 31, 2017 2016 Numerator: Net loss $ (13,492 ) $ (9,512 ) Denominator: Weighted average common shares outstanding — basic and diluted 25,043,506 24,996,921 Less: Weighted average shares of unvested restricted common stock outstanding (61,828 ) (144,988 ) Weighted average common shares outstanding used in calculating net loss per share attributable to common stockholders — basic and diluted 24,981,678 24,851,933 Net loss per share attributable to common stockholders — basic and diluted $ (0.54 ) $ (0.38 )
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 Months Ended March 31, 2017 2016 Options to purchase common stock 5,140,236 3,842,903 Unvested restricted common stock 49,260 132,428 5,189,496 3,975,331

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 2017
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 as of March 31, 2017: Year Ending December 31, 2017 (remainder of year) $ 910 2018 550 2019 505 2020 523 2021 132 Total $ 2,620

Nature of the Business and Ba24

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Accounting Policies [Abstract]
Accumulated deficit	$ 113,687		$ 100,195
Loss	13,492	$ 9,512
Cash used in operations	(18,222)	$ (9,338)
Cash, cash equivalents and marketable securities	59,144
Expected Milestones	$ 15,000

Fair Value Measurements and M25

Fair Value Measurements and Marketable Securities - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Cash equivalents:
Cash equivalents	$ 24,037	$ 30,234
Marketable securities:
Marketable securities	35,107	47,715
Total cash equivalents and marketable securities	59,144	77,949
Money Market Funds [Member]
Cash equivalents:
Cash equivalents	24,037	22,988
Commercial Paper [Member]
Marketable securities:
Marketable securities	6,744	15,814
U.S. Government Securities [Member]
Cash equivalents:
Cash equivalents		7,246
Marketable securities:
Marketable securities	14,722	18,230
Corporate Bonds [Member]
Marketable securities:
Marketable securities	13,641	13,671
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	24,037	22,988
Marketable securities:
Marketable securities	0	0
Total cash equivalents and marketable securities	24,037	22,988
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash equivalents:
Cash equivalents	24,037	22,988
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Marketable securities:
Marketable securities	0	0
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Government Securities [Member]
Cash equivalents:
Cash equivalents		0
Marketable securities:
Marketable securities	0	0
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Marketable securities:
Marketable securities	0	0
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	0	7,246
Marketable securities:
Marketable securities	35,107	47,715
Total cash equivalents and marketable securities	35,107	54,961
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash equivalents:
Cash equivalents	0	0
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Marketable securities:
Marketable securities	6,744	15,814
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Government Securities [Member]
Cash equivalents:
Cash equivalents		7,246
Marketable securities:
Marketable securities	14,722	18,230
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Marketable securities:
Marketable securities	13,641	13,671
Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	0	0
Marketable securities:
Marketable securities	0	0
Total cash equivalents and marketable securities	0	0
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash equivalents:
Cash equivalents	0	0
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Marketable securities:
Marketable securities	0	0
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Government Securities [Member]
Cash equivalents:
Cash equivalents		0
Marketable securities:
Marketable securities	0	0
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Marketable securities:
Marketable securities	$ 0	$ 0

Fair Value Measurements and M26

Fair Value Measurements and Marketable Securities - Additional Information (Detail) - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Debt Instrument Fair Value Carrying Value [Abstract]
Assets, Level 1 to Level 2 transfers, amount	$ 0	$ 0
Liabilities, Level 1 to Level 2 transfers, amount	0	0
Assets, Level 2 to Level 1 transfers, amount	0	0
Liabilities, Level 2 to Level 1 transfers, amount	$ 0	$ 0

Fair Value Measurements and M27

Fair Value Measurements and Marketable Securities - Summary of Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 35,156	$ 47,766
Gross Unrealized Gains	0	0
Gross Unrealized Loss	(49)	(51)
Fair Value	35,107	47,715
Commercial Paper (Due Within 1 Year) [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	6,744	15,814
Gross Unrealized Gains	0	0
Gross Unrealized Loss	0	0
Fair Value	6,744	15,814
U.S. Government Securities (Due Within 1 Year) [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	14,755	7,002
Gross Unrealized Gains	0	0
Gross Unrealized Loss	(33)	0
Fair Value	14,722	7,002
U.S. Government Securities (Due After 1 Year Through 2 Years) [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		11,256
Gross Unrealized Gains		0
Gross Unrealized Loss		(29)
Fair Value		11,227
Corporate Bonds (Due Within 1 Year) [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	13,657	9,182
Gross Unrealized Gains	0	0
Gross Unrealized Loss	(16)	(10)
Fair Value	$ 13,641	9,172
Corporate Bonds (Due After 1 Year Through 2 years) [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		4,512
Gross Unrealized Gains		0
Gross Unrealized Loss		(12)
Fair Value		$ 4,500

Accrued Expenses and Other Cu28

Accrued Expenses and Other Current Liabilities - Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
External research and development services	$ 2,742	$ 4,409
Payroll and payroll related costs	759	2,234
Professional services	571	433
Other	226	171
Total	$ 4,298	$ 7,247

Notes Payable - Additional Info

Notes Payable - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Second Amendment to Loan and Security Agreement [Member]
Line of Credit Facility [Line Items]
Borrowings under line of credit	$ 7,551
Principal repaid during period	491
Debt instrument final payment of accrues expenses	348
Loan and Security Agreement [Member]
Line of Credit Facility [Line Items]
Line of credit facility, periodic payment, interest	$ 115	$ 20

Notes Payable - Summary of Note

Notes Payable - Summary of Notes Payable (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Notes payable	$ 5,987	$ 6,478
Less: current portion	(2,500)	(2,361)
Notes payable, net of current portion	3,487	4,117
Debt discount, net of accretion	(25)	(30)
Accretion related to final payment	122	82
Notes payable, net of discount, long term	$ 3,584	$ 4,169

Notes Payable - Estimated Futur

Notes Payable - Estimated Future Principal Payments Due (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Notes payable	$ 5,987	$ 6,478
Loan and Security Agreement [Member]
Line of Credit Facility [Line Items]
2017 (remainder of year)	1,888
2,018	2,123
2,019	1,838
2,020	138
Notes payable	$ 5,987

Collaboration and License Agr32

Collaboration and License Agreements - Additional Information (Detail) - Bayer Healthcare LLC [Member] - Collaboration Agreement [Member] - USD ($)	1 Months Ended	3 Months Ended
	Jun. 30, 2014	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Collaborative Arrangements And Noncollaborative 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		$ 2,365,000	$ 1,264,000
Unbilled amount of research payments earned		2,365,000
Deferred revenue		$ 18,524,000		$ 17,326,000
Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential future milestone payments receivable under development and commercialization activities	$ 232,000,000

Collaboration and License Agr33

Collaboration and License Agreements - Schedule of Revenue from Collaboration Agreement (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Revenue:
Revenue	$ 3,618	$ 2,206
Collaboration Agreement [Member] | Bayer Healthcare LLC [Member]
Revenue:
Revenue	$ 3,618	$ 2,206

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jan. 02, 2017	Mar. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted in connection with employee stock purchase plan		1,008,300
Weighted average grant-date fair value of stock options granted		$ 2
Total unrecognized compensation cost related to the unvested awards		$ 8,044
Unrecognized compensation cost related to the unvested awards, weighted-average period		2 years 6 months 15 days
Restricted Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total fair value of restricted common stock vested		$ 36
2013 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future grant		0
2015 and 2013 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance		6,890,841
2015 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance		1,270,764
2015 Stock Option and Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance, percentage of number of shares of common stock outstanding		4.00%	4.00%
Additional common stock reserved for issuance			1,001,740
2015 Stock Option and Incentive Plan [Member] | Employees and Directors [Member] | Restricted Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted in connection with employee stock purchase plan		0
2015 Stock Option and Incentive Plan [Member] | Employees and Directors [Member] | Options to Purchase Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted in connection with employee stock purchase plan		1,008,300
2015 Stock Option and Incentive Plan [Member] | Non Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted in connection with employee stock purchase plan		0
2015 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance		507,020
Common stock reserved for issuance, percentage of number of shares of common stock outstanding	1.00%		1.00%
Additional common stock reserved for issuance			250,435
Stock options granted in connection with employee stock purchase plan		0
2015 Employee Stock Purchase Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional common stock reserved for issuance	750,000

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Sharebased Compensation Arrangement By Sharebased Payment Award Options Outstanding Weighted Average Exercise Price And Additional Disclosures [Abstract]
Outstanding, Number of Shares, Beginning Balance	4,185,699
Granted, Number of Shares	1,008,300
Cancelled, Number of Shares	(53,763)
Outstanding, Number of Shares, Ending Balance	5,140,236	4,185,699
Options vested and expected to vest, Number of Shares	5,140,236
Options exercisable, Number of Shares	1,803,796
Outstanding, Weighted Average Exercise Price, Beginning Balance	$ 4.26
Granted, Weighted Average Exercise Price	2
Cancelled, Weighted Average Exercise Price	9.88
Outstanding, Weighted Average Exercise Price, Ending Balance	3.75	$ 4.26
Options vested and expected to vest, Weighted Average Exercise Price	3.75
Options exercisable, Weighted Average Exercise Price	$ 3.21
Outstanding, Weighted Average Remaining Contractual Term	8 years 6 months	8 years 4 months 24 days
Options vested and expected to vest, Weighted Average Remaining Contractual Term	8 years 6 months
Options exercisable, Weighted Average Remaining Contractual Term	8 years
Outstanding, Aggregate Intrinsic Value	$ 1,281	$ 4,757
Options vested and expected to vest, Aggregate Intrinsic Value	1,281
Options exercisable, Aggregate Intrinsic Value	$ 818

Stock-Based Compensation - Su36

Stock-Based Compensation - Summary of Restricted Common Stock Activity (Detail) - Restricted Common Stock [Member]	3 Months Ended
	Mar. 31, 2017$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested restricted common stock, Number of Shares, Beginning Balance | shares	70,053
Vested, Number of Shares | shares	(20,793)
Unvested restricted common stock, Number of Shares, Ending Balance | shares	49,260
Unvested restricted common stock, Weighted Average Grant Date Fair Value, Beginning Balance | $ / shares	$ 0.88
Vested, Weighted Average Grant Date Fair Value | $ / shares	0.85
Unvested restricted common stock, Weighted Average Grant Date Fair Value, Ending Balance | $ / shares	$ 0.89

Stock-Based Compensation - Su37

Stock-Based Compensation - Summary of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 1,040	$ 533
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	380	217
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 660	$ 316

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
	Mar. 31, 2017	Mar. 31, 2016
Numerator:
Net loss	$ (13,492)	$ (9,512)
Denominator:
Weighted average common shares outstanding — basic and diluted	25,043,506	24,996,921
Less: Weighted average shares of unvested restricted common stock outstanding	(61,828)	(144,988)
Weighted average common shares outstanding used in calculating net loss per share attributable to common stockholders — basic and diluted	24,981,678	24,851,933
Net loss per share attributable to common stockholders — basic and diluted	$ (0.54)	$ (0.38)

Net Loss per Share - Schedule39

Net Loss per Share - Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share (Detail) - shares	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	5,189,496	3,975,331
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	5,140,236	3,842,903
Unvested 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	49,260	132,428

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - Laboratory and Office Facility [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Commitments and Contingencies [Line Items]
Operating leases, rental expense	$ 295	$ 171
Cambridge, MA Location [Member]
Commitments and Contingencies [Line Items]
Operating lease expiring term	2018-01
Option to extend lease term	2 years
Woburn, MA Location [Member]
Commitments and Contingencies [Line Items]
Operating lease expiring term	2021-03
Option to extend lease term	5 years

Commitments and Contingencies41

Commitments and Contingencies - Schedule of Future Minimum Lease Payments under Operating Leases (Detail) - Cambridge and Woburn Operating Leases [Member] $ in Thousands	Mar. 31, 2017USD ($)
Operating Leased Assets [Line Items]
2017 (remainder of year)	$ 910
2,018	550
2,019	505
2,020	523
2,021	132
Total	$ 2,620

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - USD ($) $ in Thousands	May 02, 2017	Apr. 18, 2017	Mar. 31, 2017
Laboratory and Office Facility [Member] | Cambridge, MA Location [Member]
Subsequent Event [Line Items]
Operating lease expiring term			2018-01
Option to extend lease term			2 years
Second Amendment to Loan and Security Agreement [Member]
Subsequent Event [Line Items]
Borrowings under line of credit			$ 7,551
Subsequent Events [Member] | Laboratory and Office Facility [Member] | Cambridge, MA Location [Member]
Subsequent Event [Line Items]
Option to extend lease term	5 years
Extended operating lease expiring term	2020-01
Subsequent Events [Member] | Lease Agreement Feburary 2018 to January 2019 [Member] | Laboratory and Office Facility [Member] | Cambridge, MA Location [Member]
Subsequent Event [Line Items]
Lease payments include base rent per month	$ 80
Subsequent Events [Member] | Lease Agreement Feburary 2019 to January 2020 [Member] | Laboratory and Office Facility [Member] | Cambridge, MA Location [Member]
Subsequent Event [Line Items]
Increased lease payments include base rent per month	$ 82
Subsequent Events [Member] | Second Amendment to Loan and Security Agreement [Member]
Subsequent Event [Line Items]
Borrowings under line of credit		$ 276
Debt issuance cost		$ 0
Accrue interest at an annual rate		3.50%
Periodic payment term of interest		6 months
Periodic payment term of interest and principal		36 months
Percentage of interest accrue on additional final payment upon maturity or early termination		6.00%