Dimension Therapeutics (DMTX) 10-Q 6 Nov 17 2017 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2017	Oct. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2,017
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	Accelerated Filer
Entity Common Stock, Shares Outstanding		25,371,591

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 14,545	$ 30,234
Marketable securities	19,171	47,715
Accounts receivable	2,779	1,885
Prepaid expenses and other current assets	6,612	5,484
Total current assets	43,107	85,318
Property and equipment, net	6,805	8,402
Deferred offering costs	206	145
Total assets	50,118	93,865
Current liabilities:
Accounts payable	1,031	2,368
Accrued expenses and other current liabilities	7,574	7,247
Deferred revenue	10,480	8,663
Notes payable	2,340	2,361
Total current liabilities	21,425	20,639
Deferred revenue, net of current portion	5,071	8,663
Notes payable, net of discount and current portion	2,847	4,169
Other liabilities	427	453
Total liabilities	29,770	33,924
Commitments and contingencies (Note 9)
Stockholders’ equity:
Preferred stock, $0.0001 par value; 5,000,000 shares authorized at September 30, 2017 and December 31, 2016; zero shares issued or outstanding at September 30, 2017 and December 31, 2016.	0	0
Common stock, $0.0001 par value; 150,000,000 shares authorized as of September 30, 2017 and December 31, 2016; 25,209,153 and 25,043,506 shares issued and outstanding as of September 30, 2017 and December 31, 2016, respectively.	2	2
Additional paid-in capital	163,316	160,185
Accumulated deficit	(142,943)	(100,195)
Accumulated other comprehensive loss	(27)	(51)
Total stockholders’ equity	20,348	59,941
Total liabilities and stockholders’ equity	$ 50,118	$ 93,865

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Sep. 30, 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	25,209,153	25,043,506
Common stock, shares outstanding	25,209,153	25,043,506

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement Of Income And Comprehensive Income [Abstract]
Revenue	$ 4,480	$ 2,762	$ 12,467	$ 7,339
Operating expenses:
Research and development	16,133	13,796	43,936	33,841
General and administrative	5,320	3,305	11,362	9,482
Total operating expenses	21,453	17,101	55,298	43,323
Loss from operations	(16,973)	(14,339)	(42,831)	(35,984)
Interest income, net	26	(26)	83	8
Net loss	$ (16,947)	$ (14,365)	$ (42,748)	$ (35,976)
Net loss per share — basic and diluted	$ (0.68)	$ (0.58)	$ (1.71)	$ (1.44)
Weighted average common shares outstanding — basic and diluted	25,060,418	24,930,348	25,015,164	24,900,773
Comprehensive loss:
Net loss	$ (16,947)	$ (14,365)	$ (42,748)	$ (35,976)
Other comprehensive loss:
Unrealized gain (loss) on marketable securities	22	(1)	(27)	(1)
Total other comprehensive gain (loss)	22	(1)	(27)	(1)
Total comprehensive loss	$ (16,925)	$ (14,366)	$ (42,775)	$ (35,977)

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities:
Net loss	$ (42,748)	$ (35,976)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	1,621	1,114
Stock-based compensation expense	2,917	2,389
Non-cash interest expense	131	65
Amortization of premium on marketable securities	39	(9)
Changes in operating assets and liabilities:
Accounts receivable	(894)	(1,664)
Prepaid expenses and other current assets	(1,128)	(160)
Accounts payable	(1,337)	(231)
Accrued expenses and other current liabilities	327	2,715
Deferred revenue	(1,775)	(932)
Other liabilities	(26)	437
Net cash used in operating activities	(42,873)	(32,252)
Cash flows from investing activities:
Purchases of marketable securities	(1,245)	(18,801)
Maturities of marketable securities	29,774	0
Purchases of property and equipment	(24)	(6,436)
Net cash provided (used) in investing activities	28,505	(25,237)
Cash flows from financing activities:
Proceeds from exercise of common stock options	214	43
Proceeds from issuance of notes payable	276	5,779
Repayment of notes payable	(1,750)	(437)
Payments of offering costs	(61)	0
Net cash provided (used) by financing activities	(1,321)	5,385
Net decrease in cash and cash equivalents	(15,689)	(52,104)
Cash and cash equivalents at beginning of period	30,234	127,047
Cash and cash equivalents at end of period	14,545	74,943
Supplemental disclosure of cash flow information:
Interest paid	145	82
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable, accrued expenses and other liabilities	$ 0	$ 66

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Dimension Therapeutics, Inc. (“we” or 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, Wilson disease, 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 September 30, 2017, the Company had an accumulated deficit of $142,943. During the three and nine months ended September 30, 2017, the Company incurred a loss of $16,947 and $42,748, respectively, and during the nine months ended September 30, 2017, the Company used $42,873 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 $33,716 as of September 30, 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. Without the Bayer milestone payments, the Company's existing capital resources, as of September 30, 2017 would not be sufficient to fund its operating expenses and capital expenditure requirements through November 6, 2018, or 12 months from issuance of the condensed consolidated financial statements. If the Company does not obtain the Bayer 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. Such contingency plans have not been finalized and are not considered probable for purposes of ASC 205-40. As neither the receipt of the Bayer milestone payments nor management's contingency plans to mitigate the risk and extend cash resources through November 6, 2018, are considered probable under ASC 205-40, substantial doubt is deemed to exist about the Company's ability to continue as a going concern. Proposed Merger with Ultragenyx Pharmaceutical Inc., a Delaware corporation (“Ultragenyx”) On October 2, 2017, the Company entered into an Agreement and Plan of Merger with Ultragenyx, and Mystic River Merger Sub Inc., a Delaware corporation and a direct, wholly-owned subsidiary of Ultragenyx (“Purchaser”), pursuant to which and upon the terms and subject to the conditions thereof, on October 10, 2017, Purchaser commenced a cash tender offer to acquire all of the shares of the Company’s common stock for a purchase price of $6.00 per share, net to the seller in cash, without interest. See Note 11 for further details. 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	9 Months Ended
	Sep. 30, 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 September 30, 2017, the condensed consolidated statements of operations and comprehensive loss for the nine months ended September 30, 2017 and 2016 and the condensed consolidated statement of cash flows for the nine months ended September 30, 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 September 30, 2017, the results of its operations for the three and nine months ended September 30, 2017 and 2016, and its statement of cash flows for the three and nine months ended September 30, 2017 and 2016. The financial data and other information disclosed in the notes related to the three and nine months ended September 30, 2017 and 2016 are unaudited. The results for the three and nine months ended September 30, 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. In May 2017, the FASB issued ASU No. 2017-09, Compensation—Stock Compensation (Topic 718) — Scope of Modification Accounting. ASU 2017-09 applies to entities that change the terms or conditions of a share-based payment award. The amendments in ASU 2017-09 include guidance on determining changes to the terms and conditions of share-based payment awards require an entity to apply modification accounting under Topic 718 unless all of the following conditions are met: (1) the fair value of the modified award is the same as the fair value of the original award immediately before the original award is modified. If the modification does not affect any of the inputs to the valuation technique that the entity uses to value the award, the entity is not required to estimate the value immediately before and after the modification; (2) the vesting conditions of the modified award are the same as the vesting conditions of the original award immediately before the original award is modified; and (3) the classification of the modified award as an equity instrument or a liability instrument is the same as the classification of the original award immediately before the original award is modified. The amendments are effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2017 and should be applied prospectively to an award modified on or after the adoption date. The Company is evaluating the impact the amendments will have on our financial positions and results of operations.

Fair Value Measurements and Mar

Fair Value Measurements and Marketable Securities	9 Months Ended
	Sep. 30, 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 September 30, 2017 using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 14,545 $ — $ — $ 14,545 Total cash equivalents 14,545 — — 14,545 Marketable securities: Commercial paper — 1,249 — 1,249 Corporate bonds — 6,691 — 6,691 U.S. government securities — 11,231 — 11,231 Total marketable securities — 19,171 — 19,171 Total cash equivalents and marketable securities $ 14,545 $ 19,171 $ — $ 33,716 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 nine months ended September 30, 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 September 30, 2017 and December 31, 2016: Fair Value Measurements as of September 30, 2017 using: Amortized Cost Gross Unrealized Gains Gross Unrealized Loss Fair Value Assets: Commercial paper (due within 1 year) $ 1,249 $ — $ — $ 1,249 Corporate bonds (due within 1 year) 6,696 — (5 ) 6,691 U.S. government securities (due within 1 year) 11,253 — (22 ) 11,231 $ 19,198 $ — $ (27 ) $ 19,171 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	9 Months Ended
	Sep. 30, 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: September 30, December 31, 2017 2016 External research and development services $ 3,992 $ 4,409 Payroll and payroll related costs 2,083 2,234 Professional services 688 433 Restructuring costs 566 — Other 245 171 $ 7,574 $ 7,247

Notes Payable

Notes Payable	9 Months Ended
	Sep. 30, 2017
Debt Disclosure [Abstract]
Notes Payable	5. Notes Payable On April 18, 2017, the Company borrowed $276 under the Second Amendment to the loan and security agreement with Silicon Valley Bank (“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. Through September 30, 2017, we had borrowed a total of $7,827 available under the loan and security agreement, as amended. During the nine months ended September 30, 2017, we made $1,750 of scheduled principal repayments. In addition, the Company accrues the related total final payments due of $365 to outstanding debt by charges to interest expense using the effective-interest method from the date of issuance through the maturity date. The effective interest rate of the outstanding debt under the Second Amendment is approximately 6.3%. As of September 30, 2017 and December 31, 2016, notes payable consist of the following: September 30, December 31, 2017 2016 Notes payable $ 5,004 $ 6,478 Less: current portion (2,340 ) (2,361 ) Notes payable, net of current portion 2,664 4,117 Debt discount, net of accretion (16 ) (30 ) Accretion related to final payment 199 82 Notes payable, net of discount, long term $ 2,847 $ 4,169 Estimated future principal payments due under the loan and security agreement with SVB as of September 30, 2017 are as follows: 2017 (remainder of year) $ 645 2018 2,215 2019 1,929 2020 215 Total $ 5,004 During the three months ended September 30, 2017 and 2016, the Company recognized $71 and $97, respectively, of interest expense related to the loan and security agreement with SVB. During the nine months ended September 30, 2017 and 2016, the Company recognized $259 and $147, respectively, of interest expense related to the loan and security agreement with SVB.

Collaboration and License Agree

Collaboration and License Agreements	9 Months Ended
	Sep. 30, 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 September 30, 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 Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Revenue: Bayer Collaboration $ 4,480 $ 2,762 $ 12,467 $ 7,339 $ 4,480 $ 2,762 $ 12,467 $ 7,339 During the three and nine months ended September 30, 2017 and 2016, research payments earned by the Company under the collaboration agreement totaled $2,779, $8,241, $1,808 and $4,391, respectively. As of September 30, 2017, the unbilled amount of earned research payments was $2,779. 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 September 30, 2017 and December 31, 2016, deferred revenue related to the collaboration agreement with Bayer totaled $15,551 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	9 Months Ended
	Sep. 30, 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 September 30, 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 1,097,881 shares available for future issuance under the 2015 Stock Option Plan. During the three and nine months ended September 30, 2017, the Company granted to employees and directors no shares of restricted common stock and options to purchase 334,000 and 1,430,300 shares of common stock. During the nine months ended September 30, 2017, the Company did not grant to non-employees any stock-based awards. 2015 Employee Stock Purchase Plan As of September 30, 2017, the total number of shares reserved and available under the 2015 Employee Stock Purchase Plan (“ESPP”) was 507,020. As of September 30, 2017, there was no activity under the ESPP. 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 1,430,300 $ 1.80 Exercised (165,647 ) $ 1.30 $ 779 Cancelled (302,880 ) $ 6.60 Outstanding as of September 30, 2017 5,147,472 $ 3.53 6.9 $ 14,123 Options vested and expected to vest as of September 30, 2017 5,147,472 $ 3.53 6.9 $ 14,123 Options exercisable as of September 30, 2017 2,858,133 $ 3.70 5.6 $ 7,352 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, 2017. The weighted average grant-date fair value of stock options granted during the three and nine months ended September 30, 2017, was $1.35 and $1.80 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 (62,375 ) $ 0.85 Unvested restricted common stock at September 30, 2017 7,678 $ 1.12 The total fair value of restricted common stock vested during the nine months ended September 30, 2017 was $374. 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 excluding expense associated with stock options modified in connection with the Company’s June 2017 restructuring efforts (Refer to Note 10) as follows: Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Research and development $ 312 $ 371 $ 974 $ 855 General and administrative 395 688 1,637 1,534 $ 707 $ 1,059 $ 2,611 $ 2,389 As of September 30, 2017, total unrecognized compensation cost related to the unvested awards was $4,877, which is expected to be recognized over a weighted average period of 2.2 years.

Net Loss per Share

Net Loss per Share	9 Months Ended
	Sep. 30, 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 Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Numerator: Net loss $ (16,947 ) $ (14,365 ) $ (42,748 ) $ (35,976 ) Denominator: Weighted average common shares outstanding — basic and diluted 25,080,616 25,033,822 25,056,012 25,024,935 Less: Weighted average shares of unvested restricted common stock outstanding (20,198 ) (103,474 ) (40,848 ) (124,162 ) Weighted average common shares outstanding used in calculating net loss per share attributable to common stockholders — basic and diluted 25,060,418 24,930,348 25,015,164 24,900,773 Net loss per share attributable to common stockholders — basic and diluted $ (0.68 ) $ (0.58 ) $ (1.71 ) $ (1.44 ) 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 Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Options to purchase common stock 5,147,472 4,161,728 5,147,472 4,161,728 Unvested restricted common stock 7,678 90,842 7,678 90,842 5,155,150 4,252,570 5,155,150 4,252,570

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2017
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	9. Commitments and Contingencies Leases 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. The Company also leases office and laboratory space under an 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 and nine months ended September 30, 2017 and 2016, the Company recognized $315, $881, $263 and $797 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 September 30, 2017: Year Ending December 31, 2017 (remainder of year) $ 303 2018 1,426 2019 1,490 2020 605 2021 132 Total $ 3,956 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 On October 11, 2017, Louis Scarantino, alleging himself to be a stockholder of the Company, filed a putative class action complaint in the United States District Court for the District of Massachusetts, captioned Louis Scarantino v. Dimension Therapeutics, Inc. et al. On October 15, 2017, another purported stockholder of the Company filed a putative class action lawsuit against the Company and its directors in the United States District Court for the District of Massachusetts, captioned Michael Rubin v. Dimension Therapeutics, Inc. et al. On October 19, 2017, another purported stockholder of the Company filed a putative class action lawsuit against the Company and its directors in the United States District Court for the District of Massachusetts, captioned Brian Eliyahou v. Dimension Therapeutics, Inc. et al. The Company believes the allegations in each of the Scarantino Complaint, the Rubin Complaint, and the Eliyahou Complaint are without merit and the Company has not recorded any amount for a loss contingency with respect to these matters. Additionally, similar cases may also be filed in connection with the proposed merger. 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. Clinical Trials On May 10, 2017, the Company announced its decision to discontinue the development of DTX101. The Company will maintain the ongoing DTX101 extension study which will monitor all patients dosed in the recently discontinued Phase 1/2 clinical trial for up to five years post dosing. Related costs will continue to be expensed as incurred. 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 September 30, 2017.

Restructuring

Restructuring	9 Months Ended
	Sep. 30, 2017
Restructuring And Related Activities [Abstract]
Restructuring	10. Restructuring On June 27, 2017, the Company announced that it completed a strategic review to focus internal efforts on the advancement of three programs to key clinical milestones. As part of a strategic restructuring, the Company expects to realize savings in operating expenses, including personnel costs, as a result of streamlining headcount by approximately 25% by the end of 2017. These actions are expected to enable the Company to focus on the timely development of gene therapies addressing unmet needs for patients suffering from inherited metabolic diseases. During the three and nine months ended September 30, 2017, the Company recorded $836 and $1,130, respectively, of restructuring-related costs in operating expense, including employee severance, benefits and related costs, as well as stock option modifications. The non-cash stock option modifications expenses for the three and nine months ended September 30, 2017 were $279 and $307, respectively. The Company does not expect to incur any additional significant costs associated with this restructuring. The following table summarizes the restructuring costs by category for the periods indicated: Three Months Ended September 30, 2017 Cash Non-Cash Total Research and development $ 124 $ 74 $ 198 General and administrative 433 205 638 $ 557 $ 279 $ 836 Nine Months Ended September 30, 2017 Cash Non-Cash Total Research and development $ 323 $ 102 $ 425 General and administrative 500 205 705 $ 823 $ 307 $ 1,130 The following table summarizes the restructuring reserve for the periods indicated: Three months ended September 30, 2017 Nine months ended September 30, 2017 Restructuring reserve beginning balance $ 258 $ — Restructuring expenses incurred during the period 557 823 Amounts paid during the period (249 ) (257 ) Restructuring reserve ending balance $ 566 $ 566

Subsequent Event

Subsequent Event	9 Months Ended
	Sep. 30, 2017
Subsequent Events [Abstract]
Subsequent Event	11. Subsequent Event On October 2, 2017, the Company entered into an Agreement and Plan of Merger (the “Merger Agreement”) with Ultragenyx Pharmaceutical Inc., a Delaware corporation (“Ultragenyx”), and Mystic River Merger Sub Inc., a Delaware corporation and a direct, wholly-owned subsidiary of Ultragenyx (“Purchaser”), pursuant to which and upon the terms and subject to the conditions thereof, on October 10, 2017, Purchaser commenced a cash tender offer (the “Offer”) to acquire all of the shares of the Company’s common stock (“Shares”) for a purchase price of $6.00 per share, net to the seller in cash (the “Offer Price”), without interest and subject to any required tax withholding. Following the consummation of the Offer, Purchaser will merge with and into the Company (the “Merger”), with the Company surviving as a wholly-owned subsidiary of Ultragenyx. The Merger will be governed by Section 251(h) of the General Corporation Law of the State of Delaware (the “DGCL”), with no stockholder vote required to consummate the Merger. In the Merger, each outstanding Share (other than Shares held by the Company, or owned by Ultragenyx, or Purchaser (other than Shares held in trust accounts, managed accounts and the like, or otherwise held in a fiduciary or agency capacity, that are beneficially owned by third parties) or held by stockholders who have properly exercised and perfected their respective demands for appraisal of such Shares in the time and manner provided in Section 262 of the DGCL and, as of the effective time of the Merger, have neither effectively withdrawn nor lost their rights to such appraisal and payment under the DGCL) will be canceled and converted automatically into the right to receive cash in an amount equal to the Offer Price, without interest and subject to any required tax withholding. The Merger Agreement may be terminated under certain circumstances, including in specified circumstances in connection with superior proposals, or if the transaction is not consummated by April 9, 2018. Upon the termination of the Merger Agreement, under specified circumstances, the Company will be required to pay Ultragenyx a termination fee of $2,850,000 and to reimburse Ultragenyx for the $2,850,000 termination fee paid by Ultragenyx on behalf of the Company in connection with the termination of the REGENXBIO Merger Agreement (as defined below). Completion of the Merger is anticipated to occur in the fourth quarter of 2017, although there can be no assurance the Merger will occur within the expected time frame or at all. The Company had previously entered into an Agreement and Plan of Merger with REGENXBIO Inc., a Delaware corporation, and Muddy Charles Acquisition Corporation, a Delaware corporation and a direct, wholly-owned subsidiary of REGENXBIO Inc., on August 24, 2017 (the “REGENXBIO Merger Agreement”), pursuant to which Muddy Charles Acquisition Corporation would merge with and into the Company, with the Company surviving the Merger as a wholly-owned subsidiary of REGENXBIO Inc. On October 2, 2017, prior to entering into the Merger Agreement with Ultragenyx, the Company terminated the REGENXBIO Merger Agreement. In connection with the termination of the REGENXBIO Merger Agreement, Ultragenyx paid REGENXBIO the $2,850,000 termination fee due under the REGENXBIO Merger Agreement on behalf of the Company.

Summary of Significant Accoun17

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 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 September 30, 2017, the condensed consolidated statements of operations and comprehensive loss for the nine months ended September 30, 2017 and 2016 and the condensed consolidated statement of cash flows for the nine months ended September 30, 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 September 30, 2017, the results of its operations for the three and nine months ended September 30, 2017 and 2016, and its statement of cash flows for the three and nine months ended September 30, 2017 and 2016. The financial data and other information disclosed in the notes related to the three and nine months ended September 30, 2017 and 2016 are unaudited. The results for the three and nine months ended September 30, 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. In May 2017, the FASB issued ASU No. 2017-09, Compensation—Stock Compensation (Topic 718) — Scope of Modification Accounting. ASU 2017-09 applies to entities that change the terms or conditions of a share-based payment award. The amendments in ASU 2017-09 include guidance on determining changes to the terms and conditions of share-based payment awards require an entity to apply modification accounting under Topic 718 unless all of the following conditions are met: (1) the fair value of the modified award is the same as the fair value of the original award immediately before the original award is modified. If the modification does not affect any of the inputs to the valuation technique that the entity uses to value the award, the entity is not required to estimate the value immediately before and after the modification; (2) the vesting conditions of the modified award are the same as the vesting conditions of the original award immediately before the original award is modified; and (3) the classification of the modified award as an equity instrument or a liability instrument is the same as the classification of the original award immediately before the original award is modified. The amendments are effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2017 and should be applied prospectively to an award modified on or after the adoption date. The Company is evaluating the impact the amendments will have on our financial positions and results of operations.

Fair Value Measurements and M18

Fair Value Measurements and Marketable Securities (Tables)	9 Months Ended
	Sep. 30, 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 September 30, 2017 using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 14,545 $ — $ — $ 14,545 Total cash equivalents 14,545 — — 14,545 Marketable securities: Commercial paper — 1,249 — 1,249 Corporate bonds — 6,691 — 6,691 U.S. government securities — 11,231 — 11,231 Total marketable securities — 19,171 — 19,171 Total cash equivalents and marketable securities $ 14,545 $ 19,171 $ — $ 33,716 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 September 30, 2017 and December 31, 2016: Fair Value Measurements as of September 30, 2017 using: Amortized Cost Gross Unrealized Gains Gross Unrealized Loss Fair Value Assets: Commercial paper (due within 1 year) $ 1,249 $ — $ — $ 1,249 Corporate bonds (due within 1 year) 6,696 — (5 ) 6,691 U.S. government securities (due within 1 year) 11,253 — (22 ) 11,231 $ 19,198 $ — $ (27 ) $ 19,171 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 Cu19

Accrued Expenses and Other Current Liabilities (Tables)	9 Months Ended
	Sep. 30, 2017
Payables And Accruals [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following: September 30, December 31, 2017 2016 External research and development services $ 3,992 $ 4,409 Payroll and payroll related costs 2,083 2,234 Professional services 688 433 Restructuring costs 566 — Other 245 171 $ 7,574 $ 7,247

Notes Payable (Tables)

Notes Payable (Tables)	9 Months Ended
	Sep. 30, 2017
Debt Disclosure [Abstract]
Summary of Notes Payable	As of September 30, 2017 and December 31, 2016, notes payable consist of the following: September 30, December 31, 2017 2016 Notes payable $ 5,004 $ 6,478 Less: current portion (2,340 ) (2,361 ) Notes payable, net of current portion 2,664 4,117 Debt discount, net of accretion (16 ) (30 ) Accretion related to final payment 199 82 Notes payable, net of discount, long term $ 2,847 $ 4,169
Estimated Future Principal Payments Due	Estimated future principal payments due under the loan and security agreement with SVB as of September 30, 2017 are as follows: 2017 (remainder of year) $ 645 2018 2,215 2019 1,929 2020 215 Total $ 5,004

Collaboration and License Agr21

Collaboration and License Agreements (Tables)	9 Months Ended
	Sep. 30, 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 Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Revenue: Bayer Collaboration $ 4,480 $ 2,762 $ 12,467 $ 7,339 $ 4,480 $ 2,762 $ 12,467 $ 7,339

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 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 1,430,300 $ 1.80 Exercised (165,647 ) $ 1.30 $ 779 Cancelled (302,880 ) $ 6.60 Outstanding as of September 30, 2017 5,147,472 $ 3.53 6.9 $ 14,123 Options vested and expected to vest as of September 30, 2017 5,147,472 $ 3.53 6.9 $ 14,123 Options exercisable as of September 30, 2017 2,858,133 $ 3.70 5.6 $ 7,352
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 (62,375 ) $ 0.85 Unvested restricted common stock at September 30, 2017 7,678 $ 1.12
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 excluding expense associated with stock options modified in connection with the Company’s June 2017 restructuring efforts (Refer to Note 10) as follows: Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Research and development $ 312 $ 371 $ 974 $ 855 General and administrative 395 688 1,637 1,534 $ 707 $ 1,059 $ 2,611 $ 2,389

Net Loss per Share (Tables)

Net Loss per Share (Tables)	9 Months Ended
	Sep. 30, 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 Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Numerator: Net loss $ (16,947 ) $ (14,365 ) $ (42,748 ) $ (35,976 ) Denominator: Weighted average common shares outstanding — basic and diluted 25,080,616 25,033,822 25,056,012 25,024,935 Less: Weighted average shares of unvested restricted common stock outstanding (20,198 ) (103,474 ) (40,848 ) (124,162 ) Weighted average common shares outstanding used in calculating net loss per share attributable to common stockholders — basic and diluted 25,060,418 24,930,348 25,015,164 24,900,773 Net loss per share attributable to common stockholders — basic and diluted $ (0.68 ) $ (0.58 ) $ (1.71 ) $ (1.44 )
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 Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Options to purchase common stock 5,147,472 4,161,728 5,147,472 4,161,728 Unvested restricted common stock 7,678 90,842 7,678 90,842 5,155,150 4,252,570 5,155,150 4,252,570

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 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 September 30, 2017: Year Ending December 31, 2017 (remainder of year) $ 303 2018 1,426 2019 1,490 2020 605 2021 132 Total $ 3,956

Restructuring (Tables)

Restructuring (Tables)	9 Months Ended
	Sep. 30, 2017
Restructuring And Related Activities [Abstract]
Summary of Restructuring Costs by Category	The following table summarizes the restructuring costs by category for the periods indicated: Three Months Ended September 30, 2017 Cash Non-Cash Total Research and development $ 124 $ 74 $ 198 General and administrative 433 205 638 $ 557 $ 279 $ 836 Nine Months Ended September 30, 2017 Cash Non-Cash Total Research and development $ 323 $ 102 $ 425 General and administrative 500 205 705 $ 823 $ 307 $ 1,130
Summary of Restructuring Reserve	The following table summarizes the restructuring reserve for the periods indicated: Three months ended September 30, 2017 Nine months ended September 30, 2017 Restructuring reserve beginning balance $ 258 $ — Restructuring expenses incurred during the period 557 823 Amounts paid during the period (249 ) (257 ) Restructuring reserve ending balance $ 566 $ 566

Nature of the Business and Ba26

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Oct. 10, 2017	Dec. 31, 2016
Nature Of Business And Basis Of Presentation [Line Items]
Accumulated deficit	$ 142,943		$ 142,943			$ 100,195
Loss	16,947	$ 14,365	42,748	$ 35,976
Cash used in operations			42,873	$ 32,252
Cash, cash equivalents and marketable securities	33,716		33,716
Expected Milestones	$ 15,000		$ 15,000
Merger Agreement [Member] | Subsequent Event [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Common stock purchase price per share					$ 6

Fair Value Measurements and M27

Fair Value Measurements and Marketable Securities - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Cash equivalents:
Cash equivalents	$ 14,545	$ 30,234
Marketable securities:
Marketable securities	19,171	47,715
Total cash equivalents and marketable securities	33,716	77,949
Money Market Funds [Member]
Cash equivalents:
Cash equivalents	14,545	22,988
Commercial Paper [Member]
Marketable securities:
Marketable securities	1,249	15,814
U.S. Government Securities [Member]
Cash equivalents:
Cash equivalents		7,246
Marketable securities:
Marketable securities	11,231	18,230
Corporate Bonds [Member]
Marketable securities:
Marketable securities	6,691	13,671
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	14,545	22,988
Marketable securities:
Marketable securities	0	0
Total cash equivalents and marketable securities	14,545	22,988
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Cash equivalents:
Cash equivalents	14,545	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	19,171	47,715
Total cash equivalents and marketable securities	19,171	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	1,249	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	11,231	18,230
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Marketable securities:
Marketable securities	6,691	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 M28

Fair Value Measurements and Marketable Securities - Additional Information (Detail) - USD ($)	Sep. 30, 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 M29

Fair Value Measurements and Marketable Securities - Summary of Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 19,198	$ 47,766
Gross Unrealized Gains	0	0
Gross Unrealized Loss	(27)	(51)
Fair Value	19,171	47,715
Commercial Paper (Due Within 1 Year) [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	1,249	15,814
Gross Unrealized Gains	0	0
Gross Unrealized Loss	0	0
Fair Value	1,249	15,814
U.S. Government Securities (Due Within 1 Year) [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	11,253	7,002
Gross Unrealized Gains	0	0
Gross Unrealized Loss	(22)	0
Fair Value	11,231	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	6,696	9,182
Gross Unrealized Gains	0	0
Gross Unrealized Loss	(5)	(10)
Fair Value	$ 6,691	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 Cu30

Accrued Expenses and Other Current Liabilities - Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
External research and development services	$ 3,992		$ 4,409
Payroll and payroll related costs	2,083		2,234
Professional services	688		433
Restructuring costs	566	$ 258
Other	245		171
Total	$ 7,574		$ 7,247

Notes Payable - Additional Info

Notes Payable - Additional Information (Detail) - USD ($) $ in Thousands	Apr. 18, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Second Amendment to Loan and Security Agreement [Member]
Line of Credit Facility [Line Items]
Borrowings under line of credit	$ 276	$ 7,827		$ 7,827
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%
Principal repaid during period				1,750
Debt instrument final payment of accrues expenses		$ 365		$ 365
Effective interest rate of the outstanding debt		6.30%		6.30%
Loan and Security Agreement [Member]
Line of Credit Facility [Line Items]
Line of credit facility, periodic payment, interest		$ 71	$ 97	$ 259	$ 147

Notes Payable - Summary of Note

Notes Payable - Summary of Notes Payable (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Notes payable	$ 5,004	$ 6,478
Less: current portion	(2,340)	(2,361)
Notes payable, net of current portion	2,664	4,117
Debt discount, net of accretion	(16)	(30)
Accretion related to final payment	199	82
Notes payable, net of discount, long term	$ 2,847	$ 4,169

Notes Payable - Estimated Futur

Notes Payable - Estimated Future Principal Payments Due (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Notes payable	$ 5,004	$ 6,478
Loan and Security Agreement [Member]
Line of Credit Facility [Line Items]
2017 (remainder of year)	645
2,018	2,215
2,019	1,929
2,020	215
Notes payable	$ 5,004

Collaboration and License Agr34

Collaboration and License Agreements - 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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 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		$ 2,779,000	$ 1,808,000	$ 8,241,000	$ 4,391,000
Unbilled amount of research payments earned		2,779,000		2,779,000
Deferred revenue		$ 15,551,000		$ 15,551,000		$ 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 Agr35

Collaboration and License Agreements - Schedule of Revenue from Collaboration Agreement (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenue:
Revenue	$ 4,480	$ 2,762	$ 12,467	$ 7,339
Collaboration Agreement [Member] | Bayer Healthcare LLC [Member]
Revenue:
Revenue	$ 4,480	$ 2,762	$ 12,467	$ 7,339

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2017USD ($)$ / sharesshares	Sep. 30, 2017USD ($)$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted in connection with employee stock purchase plan		1,430,300
Weighted average grant-date fair value of stock options granted | $ / shares	$ 1.35	$ 1.80
Total unrecognized compensation cost related to the unvested awards | $	$ 4,877	$ 4,877
Unrecognized compensation cost related to the unvested awards, weighted-average period		2 years 2 months 13 days
Restricted Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total fair value of restricted common stock vested | $		$ 374
2013 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future grant	0	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	6,890,841
2015 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance	1,097,881	1,097,881
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	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	334,000	1,430,300
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	507,020
Stock options granted in connection with employee stock purchase plan		0

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)$ / sharesshares
Sharebased Compensation Arrangement By Sharebased Payment Award Options Outstanding Weighted Average Exercise Price And Additional Disclosures [Abstract]
Outstanding, Number of Shares, Beginning Balance | shares	4,185,699
Granted, Number of Shares | shares	1,430,300
Exercised, Number of Shares | shares	(165,647)
Cancelled, Number of Shares | shares	(302,880)
Outstanding, Number of Shares, Ending Balance | shares	5,147,472	4,185,699
Options vested and expected to vest, Number of Shares | shares	5,147,472
Options exercisable, Number of Shares | shares	2,858,133
Outstanding, Weighted Average Exercise Price, Beginning Balance | $ / shares	$ 4.26
Granted, Weighted Average Exercise Price | $ / shares	1.80
Exercised, Weighted Average Exercise Price | $ / shares	1.30
Cancelled, Weighted Average Exercise Price | $ / shares	6.60
Outstanding, Weighted Average Exercise Price, Ending Balance | $ / shares	3.53	$ 4.26
Options vested and expected to vest, Weighted Average Exercise Price | $ / shares	3.53
Options exercisable, Weighted Average Exercise Price | $ / shares	$ 3.70
Outstanding, Weighted Average Remaining Contractual Term	6 years 10 months 24 days	8 years 4 months 24 days
Options vested and expected to vest, Weighted Average Remaining Contractual Term	6 years 10 months 24 days
Options exercisable, Weighted Average Remaining Contractual Term	5 years 7 months 6 days
Outstanding, Aggregate Intrinsic Value | $	$ 14,123	$ 4,757
Exercised, Aggregate Intrinsic Value | $	779
Options vested and expected to vest, Aggregate Intrinsic Value | $	14,123
Options exercisable, Aggregate Intrinsic Value | $	$ 7,352

Stock-Based Compensation - Su38

Stock-Based Compensation - Summary of Restricted Common Stock Activity (Detail) - Restricted Common Stock [Member]	9 Months Ended
	Sep. 30, 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	(62,375)
Unvested restricted common stock, Number of Shares, Ending Balance | shares	7,678
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	$ 1.12

Stock-Based Compensation - Su39

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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 707	$ 1,059	$ 2,611	$ 2,389
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	312	371	974	855
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 395	$ 688	$ 1,637	$ 1,534

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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net loss	$ (16,947)	$ (14,365)	$ (42,748)	$ (35,976)
Denominator:
Weighted average common shares outstanding — basic and diluted	25,080,616	25,033,822	25,056,012	25,024,935
Less: Weighted average shares of unvested restricted common stock outstanding	(20,198)	(103,474)	(40,848)	(124,162)
Weighted average common shares outstanding used in calculating net loss per share attributable to common stockholders — basic and diluted	25,060,418	24,930,348	25,015,164	24,900,773
Net loss per share attributable to common stockholders — basic and diluted	$ (0.68)	$ (0.58)	$ (1.71)	$ (1.44)

Net Loss per Share - Schedule41

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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	5,155,150	4,252,570	5,155,150	4,252,570
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	5,147,472	4,161,728	5,147,472	4,161,728
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	7,678	90,842	7,678	90,842

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	May 02, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Commitments and Contingencies [Line Items]
Lessee, operating lease, option to extend				Yes
Laboratory and Office Facility [Member]
Commitments and Contingencies [Line Items]
Operating leases, rental expense		$ 315	$ 263	$ 881	$ 797
Laboratory and Office Facility [Member] | Cambridge, MA Location [Member]
Commitments and Contingencies [Line Items]
Operating lease expiring term				2018-01
Option to extend lease term	5 years			2 years
Extended operating lease expiring term				2020-01
Laboratory and Office Facility [Member] | Woburn, MA Location [Member]
Commitments and Contingencies [Line Items]
Operating lease expiring term				2021-03
Option to extend lease term				5 years
Lease Agreement Feburary 2018 to January 2019 [Member] | Laboratory and Office Facility [Member] | Cambridge, MA Location [Member]
Commitments and Contingencies [Line Items]
Lease payments include base rent per month	$ 80
Lease Agreement Feburary 2019 to January 2020 [Member] | Laboratory and Office Facility [Member] | Cambridge, MA Location [Member]
Commitments and Contingencies [Line Items]
Increased lease payments include base rent per month	$ 82

Commitments and Contingencies43

Commitments and Contingencies - Schedule of Future Minimum Lease Payments under Operating Leases (Detail) - Cambridge and Woburn Operating Leases [Member] $ in Thousands	Sep. 30, 2017USD ($)
Operating Leased Assets [Line Items]
2017 (remainder of year)	$ 303
2,018	1,426
2,019	1,490
2,020	605
2,021	132
Total	$ 3,956

Restructuring - Additional Info

Restructuring - Additional Information (Detail) - USD ($) $ in Thousands	Jun. 27, 2017	Sep. 30, 2017	Sep. 30, 2017
Restructuring And Related Activities [Abstract]
Expected percentage of streamlining headcount	25.00%
Restructuring and related cost description			On June 27, 2017, the Company announced that it completed a strategic review to focus internal efforts on the advancement of three programs to key clinical milestones. As part of a strategic restructuring, the Company expects to realize savings in operating expenses, including personnel costs, as a result of streamlining headcount by approximately 25% by the end of 2017.
Restructuring-related costs recorded in operating expense		$ 836	$ 1,130
Non-cash stock option modification expenses		$ 279	$ 307

Restructuring - Summary of Rest

Restructuring - Summary of Restructuring Costs by Category (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017
Restructuring Cost And Reserve [Line Items]
Restructuring costs, Cash	$ 557	$ 823
Restructuring costs, Non-Cash	279	307
Restructuring costs	836	1,130
Research and Development [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring costs, Cash	124	323
Restructuring costs, Non-Cash	74	102
Restructuring costs	198	425
General and Administrative [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring costs, Cash	433	500
Restructuring costs, Non-Cash	205	205
Restructuring costs	$ 638	$ 705

Restructuring - Summary of Re46

Restructuring - Summary of Restructuring Reserve (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017
Restructuring And Related Activities [Abstract]
Restructuring reserve beginning balance	$ 258
Restructuring expenses incurred during the period	557	$ 823
Amounts paid during the period	(249)	(257)
Restructuring reserve ending balance	$ 566	$ 566

Subsequent Event - Additional I

Subsequent Event - Additional Information (Detail) - USD ($)	Oct. 02, 2017	Sep. 30, 2017	Oct. 10, 2017
Subsequent Event [Line Items]
Merger agreement termination description		The Merger Agreement may be terminated under certain circumstances, including in specified circumstances in connection with superior proposals, or if the transaction is not consummated by April 9, 2018. Upon the termination of the Merger Agreement, under specified circumstances, the Company will be required to pay Ultragenyx a termination fee of $2,850,000 and to reimburse Ultragenyx for the $2,850,000 termination fee paid by Ultragenyx on behalf of the Company in connection with the termination of the REGENXBIO Merger Agreement (as defined below). Completion of the Merger is anticipated to occur in the fourth quarter of 2017, although there can be no assurance the Merger will occur within the expected time frame or at all.
Merger Agreement [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Common stock purchase price per share			$ 6
Merger Agreement [Member] | Subsequent Event [Member] | Ultragenyx Pharmaceutical Inc. [Member]
Subsequent Event [Line Items]
Termination fee payable	$ 2,850,000
REGENXBIO Merger Agreement [Member] | Subsequent Event [Member] | Ultragenyx Pharmaceutical Inc. [Member]
Subsequent Event [Line Items]
Reimbursement of termination fee paid on behalf of company	2,850,000
Termination fee paid on behalf of company	$ 2,850,000