Dimension Therapeutics (DMTX) 10-Q 10 Nov 16 2016 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2016	Nov. 03, 2016
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	Q3
Trading Symbol	DMTX
Entity Registrant Name	Dimension Therapeutics, Inc.
Entity Central Index Key	1,592,288
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		25,043,506

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 74,943	$ 127,047
Marketable securities	18,809	0
Accounts receivable	1,807	143
Prepaid expenses and other current assets	2,900	2,740
Total current assets	98,459	129,930
Property and equipment, net	8,718	3,339
Total assets	107,177	133,269
Current liabilities:
Accounts payable	1,381	1,555
Accrued expenses and other current liabilities	6,968	3,715
Amounts due to related parties	0	522
Deferred revenue	7,803	6,835
Notes payable	1,973	574
Total current liabilities	18,125	13,201
Deferred revenue, net of current portion	11,770	13,670
Notes payable, net of discount and current portion	4,751	759
Other liabilities	493	56
Total liabilities	35,139	27,686
Commitments and contingencies (Note 10)
Stockholders’ equity:
Preferred stock, $0.0001 par value; 5,000,000 shares authorized at September 30, 2016 and December 31, 2015; zero shares issued or outstanding at September 30, 2016 and December 31, 2015.	0	0
Common stock, $0.0001 par value; 150,000,000 shares authorized as of September 30, 2016 and December 31, 2015; 25,043,506 and 25,008,227 shares issued and outstanding as of September 30, 2016 and December 31, 2015, respectively.	2	2
Additional paid-in capital	159,207	156,775
Accumulated deficit	(87,170)	(51,194)
Accumulated other comprehensive loss	(1)	0
Total stockholders’ equity	72,038	105,583
Total liabilities and stockholders’ equity	$ 107,177	$ 133,269

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Sep. 30, 2016	Dec. 31, 2015
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	25,008,227
Common stock, shares outstanding	25,043,506	25,008,227

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement Of Income And Comprehensive Income [Abstract]
Revenue	$ 2,762	$ 2,061	$ 7,339	$ 5,397
Operating expenses:
Research and development	13,796	10,372	33,841	25,238
General and administrative	3,305	2,334	9,482	6,115
Total operating expenses	17,101	12,706	43,323	31,353
Loss from operations	(14,339)	(10,645)	(35,984)	(25,956)
Interest income (expense), net	(26)	(22)	8	(72)
Net loss	(14,365)	(10,667)	(35,976)	(26,028)
Accretion of convertible preferred stock to redemption value	0	0	0	(23)
Net loss attributable to common stockholders	$ (14,365)	$ (10,667)	$ (35,976)	$ (26,051)
Net loss per share attributable to common stockholders — basic and diluted	$ (0.58)	$ (2.73)	$ (1.44)	$ (6.71)
Weighted average common shares outstanding — basic and diluted	24,930,348	3,907,196	24,900,773	3,880,779
Comprehensive loss:
Net loss	$ (14,365)	$ (10,667)	$ (35,976)	$ (26,028)
Other comprehensive gain (loss):
Unrealized gain (loss) on marketable securities	(1)	0	(1)	0
Total other comprehensive gain (loss)	(1)	0	(1)	0
Total comprehensive loss	$ (14,366)	$ (10,667)	$ (35,977)	$ (26,028)

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Cash flows from operating activities:
Net loss	$ (35,976)	$ (26,028)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	1,114	507
Stock-based compensation expense	2,389	886
Non-cash interest expense	65	7
Premium on marketable securities	(9)	0
Changes in operating assets and liabilities:
Accounts receivable	(1,664)	(118)
Prepaid expenses and other current assets	(160)	(1,290)
Accounts payable	(231)	54
Accrued expenses and other current liabilities and amounts due to related parties	2,715	1,633
Deferred revenue	(932)	72
Other liabilities	437	64
Net cash used in operating activities	(32,252)	(24,213)
Cash flows from investing activities:
Purchases of marketable securities	(18,801)	0
Purchases of property and equipment	(6,436)	(2,186)
Lease deposits	0	62
Net cash used in investing activities	(25,237)	(2,124)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs	0	79,219
Proceeds from exercise of common stock options	43	5
Proceeds from issuance of notes payable and common stock warrant, net of issuance costs	5,779	194
Payments of initial public offering costs	0	(1,501)
Repayment of notes payable	(437)	(248)
Net cash provided by financing activities	5,385	77,669
Net (decrease) increase in cash and cash equivalents	(52,104)	51,332
Cash and cash equivalents at beginning of period	127,047	17,913
Cash and cash equivalents at end of period	74,943	69,245
Supplemental disclosure of cash flow information:
Interest paid	82	68
Supplemental disclosure of non-cash investing and financing activities:
Accretion of convertible preferred stock to redemption value	0	23
Purchases of property and equipment included in accounts payable, accrued expenses and other liabilities	66	47
Deferred offering costs included in accounts payable and accrued expenses	$ 0	$ 1,007

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2016
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 the 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, citrullinemia type I, phenylketonuria, Wilson disease, hemophilia B, 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. The Company may not be able to obtain financing on acceptable terms, or at all, and the Company may not be able to enter into additional collaboration arrangements or obtain government grants. If the Company is unable to obtain funding, the Company could be forced to delay, reduce or eliminate its research and development programs, product portfolio expansion or commercialization efforts, which could adversely affect its business prospects. Although management continues to pursue these plans, there is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations, if at all. As of September 30, 2016, the Company had an accumulated deficit of $87,170. The Company expects to continue to generate operating losses in the foreseeable future. From our inception through September 30, 2016, we had received net proceeds of $88,895 from our sales of preferred stock, $64,620 from our initial public offering, $35,604 under the collaboration agreement with Bayer, and $7,551 from borrowings under a loan and security agreement.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2016
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, 2015. Unaudited Interim Financial Information The condensed consolidated balance sheet at December 31, 2015 was derived from audited financial statements, but does not include all disclosures required by GAAP. The accompanying condensed consolidated balance sheet as of September 30, 2016, the condensed consolidated statements of operations and comprehensive loss for the three and nine months ended September 30, 2016 and 2015 and the condensed consolidated statement of cash flows for the nine months ended September 30, 2016 and 2015 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, 2016, the results of its operations for the three and nine months ended September 30, 2016 and 2015, and its statement of cash flows for the nine months ended September 30, 2016 and 2015. The financial data and other information disclosed in the notes related to the three and nine months ended September 30, 2016 and 2015 are unaudited. The results for the three and nine months ended September 30, 2016 and 2015 are not necessarily indicative of results to be expected for the year ending December 31, 2016, 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, 2015 included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2015. 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. Marketable securities Marketable securities consist of investments with original maturities greater than ninety days. The Company has classified its investments with maturities beyond one year as short term, based on their highly liquid nature and because such marketable securities represent the investment of cash that is available for current operations. The Company considers its investment portfolio of investments available-for-sale. Accordingly, these investments are recorded at fair value, which is based on quoted market prices. Unrealized gains and losses are reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity. Realized gains and losses and declines in value judged to be other than temporary are included as a component of other income (expense), net based on the specific identification method. When determining whether a decline in value is other than temporary, the Company considers various factors, including whether the Company has the intent to sell the security, and whether it is more likely than not that the Company will be required to sell the security prior to recovery of its amortized cost basis. Fair Value Measurements Fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three categories: Level 1 — Quoted market prices in active markets for identical assets or liabilities. Level 2 — Observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3 — Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. 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 August 2014, the FASB issued ASU 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern 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

Fair Value Measurements and Mar

Fair Value Measurements and Marketable Securities	9 Months Ended
	Sep. 30, 2016
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, 2016 using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 68,354 $ — $ — $ 68,354 Commercial paper — 6,589 — 6,589 Total cash equivalents 68,354 6,589 — 74,943 Marketable securities: Commercial paper — 11,301 — 11,301 U.S. government securities — 7,508 — 7,508 Total marketable securities — 18,809 — 18,809 Total cash equivalents and marketable securities $ 68,354 $ 25,398 $ — $ 93,752 Fair Value Measurements as of December 31, 2015 using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 127,047 $ — $ — $ 127,047 Total cash equivalents $ 127,047 $ — $ — $ 127,047 During the nine months ended September 30, 2016 and the year ended December 31, 2015, there were no transfers between Level 1 and Level 2. The carrying amounts reflected in the 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, 2016: Fair Value Measurements as of September 30, 2016 using: Amortized Cost Gross Unrealized Gains Gross Unrealized Loss Fair Value Assets: Commercial paper (due within 1 year) $ 11,301 $ — $ — $ 11,301 U.S. government securities (due after 1 year through 2 years) 7,509 — (1 ) 7,508 $ 18,810 $ — $ (1 ) $ 18,809

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	9 Months Ended
	Sep. 30, 2016
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, 2016 2015 Payroll and payroll-related costs $ 1,808 $ 2,000 External research and development services 4,571 1,389 Professional fees 395 254 Other 194 72 $ 6,968 $ 3,715

Notes Payable

Notes Payable	9 Months Ended
	Sep. 30, 2016
Debt Disclosure [Abstract]
Notes Payable	5. Notes Payable In February 2016, the Company entered into a Second Amendment to the loan and security agreement (the “Second Amendment”) with Silicon Valley Bank (“SVB”). Under the Second Amendment, the Company may borrow an aggregate principal amount of up to $7,000 for certain equipment purchases. In accordance with terms of the Second Amendment, the Company is obligated to make six monthly interest-only payments. Thereafter, the Company is obligated to pay thirty-six (36) monthly installments of interest and principal. Advances under the second amendment to loan and security agreement accrue interest at an annual rate of 0.5% below the Prime Rate, which interest shall be determined by SVB on the Funding Date of the applicable Equipment Advance and shall be payable monthly in arrears. In addition, a final payment equal to 6% of the original principal amount of each advance 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. Borrowings under the loan and security agreement are secured by substantially all of our assets, except for our intellectual property. There are no financial covenants associated with the loan and security agreement. There are negative covenants restricting our activities, such as disposing of our business or certain assets, changing our business, management, ownership or business locations, incurring additional debt or liens or making payments on other debt, making certain investments and declaring dividends, acquiring or merging with another entity, engaging in transactions with affiliates or encumbering intellectual property. The obligations under the loan and security agreement are subject to acceleration upon occurrence of specified events of default, including a material adverse change in our business, operations or financial or other condition. Through September 30, 2016, the Company borrowed $5,801 under the Second Amendment in three advances and incurred $22 of related issuance costs. The issuance costs are being accreted to the principal amount of debt and being amortized to interest expense using the effective-interest method over the same period. In accordance with the terms of the Second Amendment, the borrowed loan amounts through September 30, 2016, accrues interest at an annual rate of 3.0%. 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. The effective interest rate of the outstanding debt under the Second Amendment is approximately 6.2%. As of September 30, 2016 and December 31, 2015, notes payable consist of the following: September 30, December 31, 2016 2015 Notes payable $ 6,720 $ 1,356 Less: current portion (1,973 ) (574 ) Notes payable, net of current portion 4,747 782 Debt discount, net of accretion (35 ) (23 ) Accretion related to final payment 39 — Notes payable, net of discount, long term $ 4,751 $ 759 Estimated future principal payments due under the loan and security agreement with SVB as of September 30, 2016 are as follows: 2016 (remainder of year) $ 242 2017 2,379 2018 2,123 2019 1,837 2020 139 Total $ 6,720 During the three months ended September 30, 2016 and 2015, the Company recognized $97 and $25, respectively, of interest expense related to the loan and security agreement with SVB. During the nine months ended September 30, 2016 and 2015, the Company recognized $147 and $74, 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, 2016
Collaboration And License Agreements [Abstract]
Collaboration and License Agreements	6. Collaboration and License Agreements Bayer HealthCare LLC In June 2014, the Company entered into a research and development collaboration and license agreement with Bayer for the development and commercialization of a gene therapy for the treatment of hemophilia A (the “collaboration agreement”). Under the agreement, the Company granted to Bayer an exclusive license to develop and commercialize one or more novel gene therapies for hemophilia A and a right of first notice for gene therapy treatments for hemophilia B. The Company is responsible for all research and development activities for those biologic therapeutics under the agreement prior to completion of clinical proof-of-concept (“POC”) trials, with Bayer funding the Company’s efforts and providing consulting and technological support. Upon the successful demonstration of clinical POC, the agreement requires that Bayer use commercially reasonable efforts to conduct later-stage development and commercialization of gene therapy products for treatment of hemophilia A for such therapy. Bayer will have worldwide rights to commercialize the potential future product. Financial Terms Under the terms of the agreement with Bayer, the Company received a nonrefundable, noncreditable upfront license payment of $20,000 in June 2014 and is eligible to receive development and commercialization milestone payments of up to $232,000, as well as tiered royalty payments ranging in the high single-digit to low double-digit percentages, not exceeding the mid-teens, of net sales of commercialized products resulting from the collaboration, as defined in the agreement with Bayer. Bayer will fund certain research and development services performed by the Company during the research term and will reimburse the Company for all project costs, including any third-party costs, in the performance of its obligations under the annual research plan and in accordance with the mutually agreed upon research budget. The Company determined that the deliverables under its agreement with Bayer include (i) research services to be provided over the research term, (ii) a development and commercialization license and (iii) the Company’s participation in a Joint Steering Committee (“JSC”) and a Joint Research and Development Committee (“JRDC”) to be provided over the research term. The Company determined that the development and commercialization license and involvement in the JSC and the JRDC do not have standalone value to Bayer and, therefore, are not separable from the delivery of the research services. Therefore, all deliverables under the agreement have been combined and accounted for as a single unit of accounting. Accordingly, the upfront license payment and research payments are being recognized by the Company as revenue on a straight-line basis over the estimated performance period, which approximates the 54-month research term of the agreement with Bayer that commenced in June 2014. As future research payments are earned, the Company will recognize as revenue the portion of payments equal to the percentage of the elapsed research term to the total estimated research term, with the remaining portion of consideration received being recognized over the remaining estimated performance period on a straight-line basis. The Company concluded at the outset of the arrangement that none of the future milestone payments included in the arrangement qualified as substantive milestones. Any future milestone payments will be recognized, along with the other arrangement consideration, over the remaining estimated period of performance, if any, beginning at the time a milestone payment is earned, with a cumulative catch-up being recognized for the elapsed portion of the estimated research term. As of September 30, 2016, 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, 2016 2015 2016 2015 Revenue: Bayer Collaboration $ 2,762 $ 2,061 $ 7,339 $ 5,397 $ 2,762 $ 2,061 $ 7,339 $ 5,397 During the three and nine months ended September 30, 2016 and 2015, research payments earned by the Company under the collaboration agreement totaled $1,808, $2,092, $4,391 and $5,470, respectively. As of September 30, 2016, the unbilled amount of earned research payments was $1,808. 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 statement of operations and comprehensive loss. As of September 30, 2016 and December 31, 2015, deferred revenue related to the collaboration agreement with Bayer totaled $19,573 and $20,505, respectively, which related to the upfront license payment, research services payments and to advance payments for future research services. Term and Termination The agreement with Bayer expires on a licensed treatment-by-licensed treatment and country-by-country basis until the later of ten years from the date of first commercial sale or when patent claims have expired, lapsed, been abandoned or been invalidated in such country. The Company and Bayer, through the JSC, may mutually agree to terminate the collaboration early in the event clinical development is ended as required by a regulatory authority or in light of data from any studies conducted prior the POC trial. Bayer has the unilateral right to terminate the agreement in its entirety or with respect to one or more country by providing written notice to the Company. Bayer may also terminate the agreement with notice after two unsuccessful attempts to demonstrate clinical POC in a Phase I human POC trial. Bayer may further terminate the agreement with notice in the event that, following the POC trial, there is a material safety issue with respect to a licensed product. Reciprocal termination rights under the agreement include termination for breach and termination for bankruptcy. University of Pennsylvania In May 2016, the Company entered into a Research, Collaboration and License Agreement (the “Collaboration Agreement”) with The Trustees of the University of Pennsylvania (“Penn”). The Collaboration Agreement provides the terms for the Company and Penn to collaborate with respect to the pre-clinical development of gene therapy products for the treatment of citrullinemia type I (DTX601), phenylketonuria (DTX501) and Wilson disease (DTX701) (each, a “Subfield”). Under the Collaboration Agreement, Penn granted the Company an exclusive, worldwide, royalty-bearing right and license to certain patent rights arising out of the research program, subject to certain retained rights, and a non-exclusive, worldwide, royalty-bearing right and license to certain Penn intellectual property, in each case to research, develop, make, have made, use, sell, offer for sale, commercialize and import licensed products in each Subfield for the term of the Collaboration Agreement. In May 2016, the Company paid to Penn immaterial one-time, non-refundable upfront payments recorded as research and development expense in exchange for the licenses. The Company will fund the cost of the research program in accordance with a mutually agreed-upon research budget and will be responsible for clinical development, manufacturing and commercialization of each Subfield. Research program costs will be recorded as research and development expense in the period the research services are performed. In addition, the Company would be required to make milestone payments if certain development and commercial milestones are achieved over time, as well as low to mid single-digit royalties percentages on net sales of each Subfield’s licensed products. Should they be achieved, potential development milestones through U.S. and EU regulatory acceptance total up to a maximum of $5,000 per Subfield and would be recorded as research and development expense in the periods they are achieved. As no milestones have been achieved and the agreement can be cancelled at the Company's option, no future potential milestone payments have accrued as of September 30, 2016.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2016
Related Party Transactions [Abstract]
Related Party Transactions	7. Related Party Transactions As of January 1, 2016, the Company no longer considers the University of Pennsylvania (Penn) or REGENX to be related parties, as Penn and REGENX were no longer principal shareholders and REGENX was no longer represented on the Company’s board of directors. Accordingly, expenses incurred in connection with Penn or REGENX subsequent to January 1, 2016 are no longer considered to be related party expenses. Research and development expenses for Penn were $1,498 and $4,158 for the three and nine months ended September 30, 2015. Cash payments made to the Penn during the three and nine months ended September 30, 2015 were $1,000 and $4,137, respectively. Amounts due to Penn by the Company were $522 as of December 31, 2015. Research and development expenses for REGENEX were $1,000 and $2,000 for the three and nine months ended September 30, 2015, respectively. Cash payments made to the REGENEX during the three and nine months ended September 30, 2015 was $1,000 and $2,750, respectively. Amounts due to REGENEX by the Company were $0 as of December 31, 2015.

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	8. 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, 2016, the total number of shares reserved under the 2015 Plan and the 2013 Stock Option Plan was 5,889,101 and the Company had 1,247,532 shares available for future issuance under the 2015 Stock Option Plan. During the three and nine months ended September 30, 2016, the Company granted to employees and directors no shares of restricted common stock and options to purchase 63,000 and 1,142,314 shares of common stock, respectively. During the nine months ended September 30, 2016, the Company did not grant to non-employees any stock-based awards. Stock Option Valuation The fair value of each option granted to employees and directors during the three and nine months ended September 30, 2016 and 2015 under the Company’s stock option plans has been calculated on the date of grant using the following assumptions: Three Months Ended September 30, Nine Months Ended September 30, 2016 2015 2016 2015 Expected dividend yield — — — — Expected volatility 87 - 90% 70 - 72% 83 - 90% 70 - 74% Risk free interest rate 1.0 - 1.3% 1.7% 1.0 - 1.6% 1.6 - 1.9% Expected term 5.3 - 6.0 years 6.0 - 6.1 years 5.3 - 6.0 years 5.2 - 6.9 years For options granted to nonemployees, the expected life of the option used is ten years, which is the contractual term of each such option. All other assumptions used to calculate the grant date fair value are generally consistent with the assumptions used for options granted to employees. Stock Options The following table summarizes the Company’s stock option activity since December 31, 2015: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) Outstanding as of December 31, 2015 3,097,022 $ 3.11 9.2 $ 25,618 Granted 1,142,314 $ 7.16 Exercised (35,279 ) $ 1.22 Cancelled or forfeited (42,329 ) $ 4.22 Outstanding as of September 30, 2016 4,161,728 $ 4.23 8.7 $ 16,254 Options vested and expected to vest as of September 30, 2016 4,161,728 $ 4.23 8.7 $ 16,254 Options exercisable as of September 30, 2016 1,311,850 $ 2.92 8.4 $ 6,727 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, 2016. The weighted average grant-date fair value of stock options granted during the three and nine months ended September 30, 2016, was $6.16 and $7.16 per share, respectively . 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, 2015 153,217 $ 0.87 Issued Vested (62,375 ) $ 0.85 Forfeited Unvested restricted common stock at September 30, 2016 90,842 $ 0.88 The total fair value of restricted common stock vested during the nine months ended September 30, 2016 was $498. 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 Nine Months Ended September 30, September 30, 2016 2015 2016 2015 Research and development $ 371 $ 318 $ 855 $ 487 General and administrative 688 232 1,534 399 $ 1,059 $ 550 $ 2,389 $ 886 As of September 30, 2016, total unrecognized compensation cost related to the unvested awards was $8,770, which is expected to be recognized over a weighted average period of 2.72 years.

Net Loss per Share

Net Loss per Share	9 Months Ended
	Sep. 30, 2016
Earnings Per Share [Abstract]
Net Loss per Share	9. 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, 2016 2015 2016 2015 Numerator: Net loss $ (14,365 ) $ (10,667 ) $ (35,976 ) $ (26,028 ) Accretion of convertible preferred stock to redemption value — — — (23 ) Net loss attributable to common stockholders $ (14,365 ) $ (10,667 ) $ (35,976 ) $ (26,051 ) Denominator: Weighted average common shares outstanding — basic and diluted 25,033,822 4,093,836 25,024,935 4,077,551 Less: Weighted average shares of unvested restricted common stock outstanding (103,474 ) (186,640 ) (124,162 ) (196,772 ) Weighted average common shares outstanding used in calculating net loss per share attributable to common stockholders — basic and diluted 24,930,348 3,907,196 24,900,773 3,880,779 Net loss per share attributable to common stockholders — basic and diluted $ (0.58 ) $ (2.73 ) $ (1.44 ) $ (6.71 ) The Company’s potential dilutive securities, which include convertible preferred stock, stock options, unvested restricted common stock and a warrant to purchase common stock, have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders 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, 2016 2015 2016 2015 Convertible preferred stock (as converted to common stock) — 15,286,968 — 15,286,968 Options to purchase common stock 4,161,728 3,021,324 4,161,728 3,021,324 Unvested restricted common stock 90,842 174,008 90,842 174,008 Warrant for the purchase of common stock — 11,973 — 11,973 4,252,570 18,494,273 4,252,570 18,494,273

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2016
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	10. Commitments and Contingencies Leases The Company leases office and laboratory space under various operating lease agreements for its Cambridge, MA location that, as amended, expires January 2018. The Company also operating lease agreement for its Woburn, MA location that expires March 2021 . The Company took occupancy of the Woburn, MA facility on April 1, 2016. The following table summarizes the total rent expense recorded during the three and nine months ended September 30, 2016 and 2015, respectively: Three Months Ended Nine Months Ended September 30, September 30, 2016 2015 2016 2015 (in thousands) (in thousands) Cambridge, MA $ 171 $ 171 $ 512 $ 563 Woburn, MA 92 — 185 — Total rent expense $ 263 $ 171 $ 697 $ 563 The following table summarizes the future minimum lease payments under the Cambridge and Woburn operating leases as of September 30, 2016: Year Ending December 31, 2016 (remainder of year) $ 293 2017 1,205 2018 549 2019 506 2020 523 2021 132 Total $ 3,208 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 September 30, 2016.

Summary of Significant Accoun16

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2016
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, 2015.
Unaudited Interim Financial Information	Unaudited Interim Financial Information The condensed consolidated balance sheet at December 31, 2015 was derived from audited financial statements, but does not include all disclosures required by GAAP. The accompanying condensed consolidated balance sheet as of September 30, 2016, the condensed consolidated statements of operations and comprehensive loss for the three and nine months ended September 30, 2016 and 2015 and the condensed consolidated statement of cash flows for the nine months ended September 30, 2016 and 2015 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, 2016, the results of its operations for the three and nine months ended September 30, 2016 and 2015, and its statement of cash flows for the nine months ended September 30, 2016 and 2015. The financial data and other information disclosed in the notes related to the three and nine months ended September 30, 2016 and 2015 are unaudited. The results for the three and nine months ended September 30, 2016 and 2015 are not necessarily indicative of results to be expected for the year ending December 31, 2016, 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, 2015 included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2015.
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.
Marketable securities	Marketable securities Marketable securities consist of investments with original maturities greater than ninety days. The Company has classified its investments with maturities beyond one year as short term, based on their highly liquid nature and because such marketable securities represent the investment of cash that is available for current operations. The Company considers its investment portfolio of investments available-for-sale. Accordingly, these investments are recorded at fair value, which is based on quoted market prices. Unrealized gains and losses are reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity. Realized gains and losses and declines in value judged to be other than temporary are included as a component of other income (expense), net based on the specific identification method. When determining whether a decline in value is other than temporary, the Company considers various factors, including whether the Company has the intent to sell the security, and whether it is more likely than not that the Company will be required to sell the security prior to recovery of its amortized cost basis.
Fair Value Measurements	Fair Value Measurements Fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three categories: Level 1 — Quoted market prices in active markets for identical assets or liabilities. Level 2 — Observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3 — Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.
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 August 2014, the FASB issued ASU 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern 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

Fair Value Measurements and M17

Fair Value Measurements and Marketable Securities (Tables)	9 Months Ended
	Sep. 30, 2016
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, 2016 using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 68,354 $ — $ — $ 68,354 Commercial paper — 6,589 — 6,589 Total cash equivalents 68,354 6,589 — 74,943 Marketable securities: Commercial paper — 11,301 — 11,301 U.S. government securities — 7,508 — 7,508 Total marketable securities — 18,809 — 18,809 Total cash equivalents and marketable securities $ 68,354 $ 25,398 $ — $ 93,752 Fair Value Measurements as of December 31, 2015 using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 127,047 $ — $ — $ 127,047 Total cash equivalents $ 127,047 $ — $ — $ 127,047
Summary of Marketable Securities	The following tables summarize the Company’s marketable securities as of September 30, 2016: Fair Value Measurements as of September 30, 2016 using: Amortized Cost Gross Unrealized Gains Gross Unrealized Loss Fair Value Assets: Commercial paper (due within 1 year) $ 11,301 $ — $ — $ 11,301 U.S. government securities (due after 1 year through 2 years) 7,509 — (1 ) 7,508 $ 18,810 $ — $ (1 ) $ 18,809

Accrued Expenses and Other Cu18

Accrued Expenses and Other Current Liabilities (Tables)	9 Months Ended
	Sep. 30, 2016
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, 2016 2015 Payroll and payroll-related costs $ 1,808 $ 2,000 External research and development services 4,571 1,389 Professional fees 395 254 Other 194 72 $ 6,968 $ 3,715

Notes Payable (Tables)

Notes Payable (Tables)	9 Months Ended
	Sep. 30, 2016
Debt Disclosure [Abstract]
Summary of Notes Payable	As of September 30, 2016 and December 31, 2015, notes payable consist of the following: September 30, December 31, 2016 2015 Notes payable $ 6,720 $ 1,356 Less: current portion (1,973 ) (574 ) Notes payable, net of current portion 4,747 782 Debt discount, net of accretion (35 ) (23 ) Accretion related to final payment 39 — Notes payable, net of discount, long term $ 4,751 $ 759
Estimated Future Principal Payments Due	Estimated future principal payments due under the loan and security agreement with SVB as of September 30, 2016 are as follows: 2016 (remainder of year) $ 242 2017 2,379 2018 2,123 2019 1,837 2020 139 Total $ 6,720

Collaboration and License Agr20

Collaboration and License Agreements (Tables)	9 Months Ended
	Sep. 30, 2016
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, 2016 2015 2016 2015 Revenue: Bayer Collaboration $ 2,762 $ 2,061 $ 7,339 $ 5,397 $ 2,762 $ 2,061 $ 7,339 $ 5,397

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Assumptions Used in Fair Value of Options Granted to Employees and Directors Under Stock Option Plans	The fair value of each option granted to employees and directors during the three and nine months ended September 30, 2016 and 2015 under the Company’s stock option plans has been calculated on the date of grant using the following assumptions: Three Months Ended September 30, Nine Months Ended September 30, 2016 2015 2016 2015 Expected dividend yield — — — — Expected volatility 87 - 90% 70 - 72% 83 - 90% 70 - 74% Risk free interest rate 1.0 - 1.3% 1.7% 1.0 - 1.6% 1.6 - 1.9% Expected term 5.3 - 6.0 years 6.0 - 6.1 years 5.3 - 6.0 years 5.2 - 6.9 years
Summary of Stock Option Activity	The following table summarizes the Company’s stock option activity since December 31, 2015: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) Outstanding as of December 31, 2015 3,097,022 $ 3.11 9.2 $ 25,618 Granted 1,142,314 $ 7.16 Exercised (35,279 ) $ 1.22 Cancelled or forfeited (42,329 ) $ 4.22 Outstanding as of September 30, 2016 4,161,728 $ 4.23 8.7 $ 16,254 Options vested and expected to vest as of September 30, 2016 4,161,728 $ 4.23 8.7 $ 16,254 Options exercisable as of September 30, 2016 1,311,850 $ 2.92 8.4 $ 6,727
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, 2015 153,217 $ 0.87 Issued Vested (62,375 ) $ 0.85 Forfeited Unvested restricted common stock at September 30, 2016 90,842 $ 0.88
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 Nine Months Ended September 30, September 30, 2016 2015 2016 2015 Research and development $ 371 $ 318 $ 855 $ 487 General and administrative 688 232 1,534 399 $ 1,059 $ 550 $ 2,389 $ 886

Net Loss per Share (Tables)

Net Loss per Share (Tables)	9 Months Ended
	Sep. 30, 2016
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, 2016 2015 2016 2015 Numerator: Net loss $ (14,365 ) $ (10,667 ) $ (35,976 ) $ (26,028 ) Accretion of convertible preferred stock to redemption value — — — (23 ) Net loss attributable to common stockholders $ (14,365 ) $ (10,667 ) $ (35,976 ) $ (26,051 ) Denominator: Weighted average common shares outstanding — basic and diluted 25,033,822 4,093,836 25,024,935 4,077,551 Less: Weighted average shares of unvested restricted common stock outstanding (103,474 ) (186,640 ) (124,162 ) (196,772 ) Weighted average common shares outstanding used in calculating net loss per share attributable to common stockholders — basic and diluted 24,930,348 3,907,196 24,900,773 3,880,779 Net loss per share attributable to common stockholders — basic and diluted $ (0.58 ) $ (2.73 ) $ (1.44 ) $ (6.71 )
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, 2016 2015 2016 2015 Convertible preferred stock (as converted to common stock) — 15,286,968 — 15,286,968 Options to purchase common stock 4,161,728 3,021,324 4,161,728 3,021,324 Unvested restricted common stock 90,842 174,008 90,842 174,008 Warrant for the purchase of common stock — 11,973 — 11,973 4,252,570 18,494,273 4,252,570 18,494,273

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2016
Commitments And Contingencies Disclosure [Abstract]
Summary of Total Rent Expense	The following table summarizes the total rent expense recorded during the three and nine months ended September 30, 2016 and 2015, respectively: Three Months Ended Nine Months Ended September 30, September 30, 2016 2015 2016 2015 (in thousands) (in thousands) Cambridge, MA $ 171 $ 171 $ 512 $ 563 Woburn, MA 92 — 185 — Total rent expense $ 263 $ 171 $ 697 $ 563
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, 2016: Year Ending December 31, 2016 (remainder of year) $ 293 2017 1,205 2018 549 2019 506 2020 523 2021 132 Total $ 3,208

Nature of the Business and Ba24

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	39 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Nature Of Business And Basis Of Presentation [Line Items]
Accumulated deficit	$ 87,170	$ 51,194
Net proceeds from sale of preferred stock	88,895
Net proceeds from initial public offering	64,620
Loan and Security Agreement [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Net proceeds from borrowings	7,551
Collaboration Agreement [Member] | Bayer Healthcare LLC [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Net proceeds from collaboration agreement	$ 35,604

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	Sep. 30, 2016	Dec. 31, 2015
Cash equivalents:
Cash equivalents	$ 74,943	$ 127,047
Marketable securities:
Marketable securities	18,809
Total cash equivalents and marketable securities	93,752
Money Market Funds [Member]
Cash equivalents:
Cash equivalents	68,354	127,047
Commercial Paper [Member]
Cash equivalents:
Cash equivalents	6,589
Marketable securities:
Marketable securities	11,301
U.S. Government Securities [Member]
Marketable securities:
Marketable securities	7,508
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	68,354	127,047
Marketable securities:
Marketable securities	0
Total cash equivalents and marketable securities	68,354
Level 1 [Member] | Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	68,354	127,047
Level 1 [Member] | Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	0
Marketable securities:
Marketable securities	0
Level 1 [Member] | U.S. Government Securities [Member] | Fair Value, Measurements, Recurring [Member]
Marketable securities:
Marketable securities	0
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	6,589	0
Marketable securities:
Marketable securities	18,809
Total cash equivalents and marketable securities	25,398
Level 2 [Member] | Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	0	0
Level 2 [Member] | Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	6,589
Marketable securities:
Marketable securities	11,301
Level 2 [Member] | U.S. Government Securities [Member] | Fair Value, Measurements, Recurring [Member]
Marketable securities:
Marketable securities	7,508
Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	0	0
Marketable securities:
Marketable securities	0
Total cash equivalents and marketable securities	0
Level 3 [Member] | Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	0	$ 0
Level 3 [Member] | Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	0
Marketable securities:
Marketable securities	0
Level 3 [Member] | U.S. Government Securities [Member] | Fair Value, Measurements, Recurring [Member]
Marketable securities:
Marketable securities	$ 0

Fair Value Measurements and M26

Fair Value Measurements and Marketable Securities - Additional Information (Detail) - USD ($)	Sep. 30, 2016	Dec. 31, 2015
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) $ in Thousands	Sep. 30, 2016USD ($)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 18,810
Gross Unrealized Gains	0
Gross Unrealized Loss	(1)
Fair Value	18,809
Commercial Paper (Due Within 1 Year) [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	11,301
Gross Unrealized Gains	0
Gross Unrealized Loss	0
Fair Value	11,301
U.S. Government Securities (Due After 1 Year Through 2 Years) [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	7,509
Gross Unrealized Gains	0
Gross Unrealized Loss	(1)
Fair Value	$ 7,508

Accrued Expenses and Other Cu28

Accrued Expenses and Other Current Liabilities - Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Payables And Accruals [Abstract]
Payroll and payroll-related costs	$ 1,808	$ 2,000
External research and development services	4,571	1,389
Professional fees	395	254
Other	194	72
Total	$ 6,968	$ 3,715

Notes Payable - Additional Info

Notes Payable - Additional Information (Detail) - USD ($)	Feb. 29, 2016	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Second Amendment to Loan and Security Agreement [Member]
Line of Credit Facility [Line Items]
Maximum borrowings under line of credit	$ 7,000,000
Periodic payment term of interest				6 months
Periodic payment term of interest and principal				36 months
Accrue interest at annual rate below prime rate, to be determined on date of drawdown	0.50%
Percentage of interest accrue on additional final payment upon maturity or early termination	6.00%
Borrowings under line of credit		$ 5,801,000		$ 5,801,000
Debt issuance cost		$ 22,000		$ 22,000
Accrues interest at an annual rate		3.00%		3.00%
Debt instrument final payment of accrues expenses		$ 348,000		$ 348,000
Effective interest rate of the outstanding debt		6.20%		6.20%
Loan and Security Agreement [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Periodic Payment, Interest		$ 97,000	$ 25,000	$ 147,000	$ 74,000

Notes Payable - Summary of Note

Notes Payable - Summary of Notes Payable (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Debt Disclosure [Abstract]
Notes payable	$ 6,720	$ 1,356
Less: current portion	(1,973)	(574)
Notes payable, net of current portion	4,747	782
Debt discount, net of accretion	(35)	(23)
Accretion related to final payment	39	0
Notes payable, net of discount, long term	$ 4,751	$ 759

Notes Payable - Estimated Futur

Notes Payable - Estimated Future Principal Payments Due (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Line of Credit Facility [Line Items]
Notes payable	$ 6,720	$ 1,356
Loan and Security Agreement [Member]
Line of Credit Facility [Line Items]
2016 (remainder of year)	242
2,017	2,379
2,018	2,123
2,019	1,837
2,020	139
Notes payable	$ 6,720

Collaboration and License Agr32

Collaboration and License Agreements - Additional Information (Detail) - Collaboration Agreement [Member] - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 30, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
University of Pennsylvania [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential milestone payment accrued		$ 0		$ 0
Maximum [Member] | University of Pennsylvania [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development expense expected to be incurred per subfield on clinical development		5,000,000		5,000,000
Bayer Healthcare LLC [Member]
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		1,808,000	$ 2,092,000	4,391,000	$ 5,470,000
Unbilled amount of research payments earned		1,808,000		1,808,000
Deferred revenue		$ 19,573,000		$ 19,573,000		$ 20,505,000
Bayer Healthcare LLC [Member] | 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		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Revenue:
Revenue	$ 2,762	$ 2,061	$ 7,339	$ 5,397
Collaboration Agreement [Member] | Bayer Healthcare LLC [Member]
Revenue:
Revenue	$ 2,762	$ 2,061	$ 7,339	$ 5,397

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2015	Sep. 30, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Amounts due to related party			$ 0	$ 522
University of Pennsylvania [Member]
Related Party Transaction [Line Items]
Related party expenses	$ 1,498	$ 4,158
Cash payment to related party	1,000	4,137
Amounts due to related party				522
REGENX [Member]
Related Party Transaction [Line Items]
Related party expenses	1,000	2,000
Cash payment to related party	$ 1,000	$ 2,750
Amounts due to related party				$ 0

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2016	Sep. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted in connection with employee stock purchase plan		1,142,314
Weighted average grant-date fair value of stock options granted	$ 6.16	$ 7.16
Total unrecognized compensation cost related to the unvested awards	$ 8,770	$ 8,770
Unrecognized compensation cost related to the unvested awards, weighted-average period		2 years 8 months 19 days
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
Total fair value of restricted common stock vested		$ 498
Non Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term		10 years
2015 and 2013 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance	5,889,101	5,889,101
2013 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future grant			0
2015 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance	1,247,532	1,247,532
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	63,000	1,142,314
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

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Assumptions Used in Fair Value of Options Granted to Employees and Directors Under Stock Option Plans (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%	0.00%	0.00%
Expected volatility, Minimum	87.00%	70.00%	83.00%	70.00%
Expected volatility, Maximum	90.00%	72.00%	90.00%	74.00%
Risk free interest rate		1.70%
Risk free interest rate, Minimum	1.00%		1.00%	1.60%
Risk free interest rate, Maximum	1.30%		1.60%	1.90%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	5 years 3 months 18 days	6 years	5 years 3 months 18 days	5 years 2 months 12 days
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	6 years	6 years 1 month 6 days	6 years	6 years 10 months 24 days

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, 2016USD ($)$ / sharesshares	Dec. 31, 2015USD ($)$ / 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	3,097,022
Granted, Number of Shares | shares	1,142,314
Exercised, Number of Shares | shares	(35,279)
Cancelled or forfeited, Number of Shares | shares	(42,329)
Outstanding, Number of Shares, Ending Balance | shares	4,161,728	3,097,022
Options vested and expected to vest, Number of Shares | shares	4,161,728
Options exercisable, Number of Shares | shares	1,311,850
Outstanding, Weighted Average Exercise Price, Beginning Balance | $ / shares	$ 3.11
Granted, Weighted Average Exercise Price | $ / shares	7.16
Exercised, Weighted Average Exercise Price | $ / shares	1.22
Cancelled or forfeited, Weighted Average Exercise Price | $ / shares	4.22
Outstanding, Weighted Average Exercise Price, Ending Balance | $ / shares	4.23	$ 3.11
Options vested and expected to vest, Weighted Average Exercise Price | $ / shares	4.23
Options exercisable, Weighted Average Exercise Price | $ / shares	$ 2.92
Outstanding, Weighted Average Remaining Contractual Term	8 years 8 months 12 days	9 years 2 months 12 days
Options vested and expected to vest, Weighted Average Remaining Contractual Term	8 years 8 months 12 days
Options exercisable, Weighted Average Remaining Contractual Term	8 years 4 months 24 days
Outstanding, Aggregate Intrinsic Value | $	$ 16,254	$ 25,618
Options vested and expected to vest, Aggregate Intrinsic Value | $	16,254
Options exercisable, Aggregate Intrinsic Value | $	$ 6,727

Stock-Based Compensation - Su38

Stock-Based Compensation - Summary of Restricted Common Stock Activity (Detail) - Restricted Common Stock [Member]	9 Months Ended
	Sep. 30, 2016$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested restricted common stock, Number of Shares, Beginning Balance	153,217
Issued, Number of Shares	0
Vested, Number of Shares	(62,375)
Forfeited, Number of Shares	0
Unvested restricted common stock, Number of Shares, Ending Balance	90,842
Unvested restricted common stock, Weighted Average Grant Date Fair Value, Beginning Balance | $ / shares	$ 0.87
Vested, Weighted Average Grant Date Fair Value | $ / shares	0.85
Unvested restricted common stock, Weighted Average Grant Date Fair Value, Ending Balance | $ / shares	$ 0.88

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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 1,059	$ 550	$ 2,389	$ 886
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	371	318	855	487
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 688	$ 232	$ 1,534	$ 399

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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Numerator:
Net loss	$ (14,365)	$ (10,667)	$ (35,976)	$ (26,028)
Accretion of convertible preferred stock to redemption value	0	0	0	(23)
Net loss attributable to common stockholders	$ (14,365)	$ (10,667)	$ (35,976)	$ (26,051)
Denominator:
Weighted average common shares outstanding — basic and diluted	25,033,822	4,093,836	25,024,935	4,077,551
Less: Weighted average shares of unvested restricted common stock outstanding	(103,474)	(186,640)	(124,162)	(196,772)
Weighted average common shares outstanding used in calculating net loss per share attributable to common stockholders — basic and diluted	24,930,348	3,907,196	24,900,773	3,880,779
Net loss per share attributable to common stockholders — basic and diluted	$ (0.58)	$ (2.73)	$ (1.44)	$ (6.71)

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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	4,252,570	18,494,273	4,252,570	18,494,273
Convertible Preferred Stock (as Converted to Common Stock) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	0	15,286,968	0	15,286,968
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	4,161,728	3,021,324	4,161,728	3,021,324
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	90,842	174,008	90,842	174,008
Warrant for the Purchase of Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share		11,973		11,973

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - Laboratory and Office Facility [Member]	9 Months Ended
	Sep. 30, 2016
Cambridge, MA Location [Member]
Commitments and Contingencies [Line Items]
Operating lease expiration period	2018-01
Woburn, MA Location [Member]
Commitments and Contingencies [Line Items]
Operating lease expiration period	2021-03
Lease start date	Apr. 1, 2016

Commitments and Contingencies43

Commitments and Contingencies - Summary of Total Rent Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Operating Leased Assets [Line Items]
Total rent expense	$ 263	$ 171	$ 697	$ 563
Cambridge, MA Location [Member]
Operating Leased Assets [Line Items]
Total rent expense	171	171	512	563
Woburn, MA Location [Member]
Operating Leased Assets [Line Items]
Total rent expense	$ 92	$ 0	$ 185	$ 0

Commitments and Contingencies44

Commitments and Contingencies - Schedule of Future Minimum Lease Payments under Operating Leases (Detail) - Cambridge and Woburn Operating Leases [Member] $ in Thousands	Sep. 30, 2016USD ($)
Operating Leased Assets [Line Items]
2016 (remainder of year)	$ 293
2,017	1,205
2,018	549
2,019	506
2,020	523
2,021	132
Total	$ 3,208