Adverum Biotechnologies (ADVM) 10-K 6 Mar 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Feb. 28, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Trading Symbol	ADVM
Entity Registrant Name	Adverum Biotechnologies, Inc.
Entity Central Index Key	1,501,756
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		62,173,278
Entity Public Float			$ 87.8

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 70,519	$ 222,170
Short-term investments	119,966
Receivable from collaborative partner		886
Prepaid expenses and other current assets	3,256	2,218
Total current assets	193,741	225,274
Property and equipment, net	3,024	4,169
Deposit and other non-current assets	140	140
Intangible asset	5,000	5,000
Total assets	201,905	234,583
Current liabilities:
Accounts payable	1,731	1,474
Accrued expenses and other current liabilities	6,964	6,476
Deferred rent, current portion	129	96
Deferred revenue, current portion	1,850	1,850
Total current liabilities	10,674	9,896
Long-term liabilities:
Deferred rent, net of current portion	222	352
Deferred revenue, net of current portion	5,250	7,099
Deferred tax liability, non-current	1,250	1,250
Other non-current liabilities	481	386
Total liabilities	17,877	18,983
Commitments and contingencies (Note 11)
Stockholders’ equity:
Preferred stock, $0.0001 par value, 5,000,000 shares authorized; no shares issued and outstanding
Common stock, $0.0001 par value, 300,000,000 shares authorized at December 31, 2017 and 2016; 49,015,339 and 41,805,009 shares issued and outstanding at December 31, 2017 and 2016, respectively	5	4
Additional paid-in capital	439,048	413,518
Accumulated other comprehensive loss	(963)	(7)
Accumulated deficit	(254,062)	(197,915)
Total stockholders’ equity	184,028	215,600
Total liabilities and stockholders’ equity	$ 201,905	$ 234,583

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	49,015,339	41,805,009
Common stock, shares outstanding	49,015,339	41,805,009

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Collaboration and license revenue	$ 1,849	$ 1,455	$ 2,319
Operating expenses:
Research and development	39,839	31,670	25,462
General and administrative	20,857	24,355	22,107
Impairment of goodwill and intangible assets		60,714
Restructuring charges			2,573
Total operating expenses	60,696	116,739	50,142
Operating loss	(58,847)	(115,284)	(47,823)
Other income:
Other income, net	2,700	762	370
Total other income, net	2,700	762	370
Net loss before income taxes	(56,147)	(114,522)	(47,453)
Income tax benefit	0	(775)	0
Net loss attributable to common stockholders	(56,147)	(113,747)	(47,453)
Other comprehensive loss:
Net unrealized gain (loss) on marketable securities	(182)	6	(6)
Foreign currency translation adjustment	(774)	(2)	(15)
Comprehensive loss	$ (57,103)	$ (113,743)	$ (47,474)
Net loss per share attributable to common stockholders-basic and diluted	$ (1.29)	$ (3.14)	$ (1.86)
Weighted-average common shares outstanding-basic and diluted	43,661	36,246	25,479

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2014	$ 149,483	$ 2	$ 186,186	$ 10	$ (36,715)
Balance, shares at Dec. 31, 2014		22,754,037
Issuance of common stock net of issuance costs	138,954	$ 1	138,953
Issuance of common stock net of issuance costs, shares		2,599,375
Issuance of common stock warrants in consideration for services and in connection with financing arrangement	218		218
Stock-based compensation expense	11,505		11,505
Common stock issued upon exercise of stock options	188		188
Common stock issued upon exercise of stock options, shares		399,434
Common stock issued under employee stock purchase plan	145		145
Common stock issued under employee stock purchase plan, shares		19,577
Common stock issued upon release of restricted stock units, shares		132,397
Restricted stock surrendered for taxes	(427)		(427)
Restricted stock surrendered for taxes, shares		(46,098)
Net unrealized gain (loss) on marketable securities	(6)			(6)
Foreign currency translation adjustment	(15)			(15)
Net loss	(47,453)				(47,453)
Balance at Dec. 31, 2015	252,592	$ 3	336,768	(11)	(84,168)
Balance, shares at Dec. 31, 2015		25,858,722
Issuance of common stock in consideration of acquisition	64,845	$ 1	64,844
Issuance of common stock in consideration of acquisition, shares		14,087,246
Remeasurement of contingent common stock warrant in consideration for services	8		8
Issuance of common stock warrants in consideration for services and in connection with financing arrangement	26		26
Stock-based compensation expense	11,416		11,416
Common stock issued upon exercise of stock options	763		763
Common stock issued upon exercise of stock options, shares		1,525,687
Common stock issued under employee stock purchase plan	186		186
Common stock issued under employee stock purchase plan, shares		56,696
Common stock issued upon release of restricted stock units, shares		385,524
Restricted stock surrendered for taxes	(493)		(493)
Restricted stock surrendered for taxes, shares		(108,866)
Net unrealized gain (loss) on marketable securities	6			6
Foreign currency translation adjustment	(2)			(2)
Net loss	(113,747)				(113,747)
Balance at Dec. 31, 2016	215,600	$ 4	413,518	(7)	(197,915)
Balance, shares at Dec. 31, 2016		41,805,009
Issuance of common stock net of issuance costs	16,519	$ 1	16,518
Issuance of common stock net of issuance costs, shares		5,130,339
Remeasurement of contingent common stock warrant in consideration for services	60		60
Stock-based compensation expense	8,723		8,723
Common stock issued upon exercise of stock options	367		367
Common stock issued upon exercise of stock options, shares		1,808,696
Common stock issued under employee stock purchase plan	175		175
Common stock issued under employee stock purchase plan, shares		74,642
Common stock issued upon release of restricted stock units, shares		307,610
Restricted stock surrendered for taxes	(313)		(313)
Restricted stock surrendered for taxes, shares		(110,957)
Net unrealized gain (loss) on marketable securities	(182)			(182)
Foreign currency translation adjustment	(774)			(774)
Net loss	(56,147)				(56,147)
Balance at Dec. 31, 2017	$ 184,028	$ 5	$ 439,048	$ (963)	$ (254,062)
Balance, shares at Dec. 31, 2017		49,015,339

Consolidated Statements of Sto6

Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2015
Statement Of Stockholders Equity [Abstract]
Issuance costs	$ 230	$ 11,099

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (56,147)	$ (113,747)	$ (47,453)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	2,096	1,603	812
Stock-based compensation expense	8,723	11,416	11,505
Amortization of premium on marketable securities	593		780
Impairment of goodwill and intangible assets		60,714
Non-cash research and development expense	60	8	218
Other	10		21
Changes in operating assets and liabilities:
Accounts receivable, net	886	(437)	(6)
Prepaid expenses and other current assets	(491)	71	(531)
Deposit and other long-term assets			(3)
Accounts payable	333	(111)	(312)
Accrued expenses and other current liabilities	487	(190)	724
Restructuring liabilities	(25)	(988)	1,013
Deferred revenue	(1,849)	3,360	(2,313)
Deferred rent	(97)	(65)	207
Net cash used in operating activities	(45,421)	(38,366)	(35,338)
Cash flows from investing activities:
Purchases of marketable securities	(209,787)		(88,427)
Sales of marketable securities	1,003
Maturities of marketable securities	87,596	37,738	49,850
Purchases of property and equipment	(1,016)	(2,412)	(2,992)
Cash acquired in business acquisition		3,449
Net cash (used in) provided by investing activities	(122,204)	38,775	(41,569)
Cash flows from financing activities:
Proceeds from offering of common stock, net of issuance costs	16,519		138,954
Proceeds from issuance of common stock pursuant to option exercises	367	763	188
Taxes paid related to net share settlement of restricted stock units	(313)	(493)	(427)
Proceeds from employee stock purchase plan	175	186	145
Proceeds from a financing arrangement		100
Net cash provided by financing activities	16,748	556	138,860
Effect of foreign currency exchange rate on cash and cash equivalents	(774)	(143)	(9)
Net (decrease) increase in cash and cash equivalents	(151,651)	822	61,944
Cash and cash equivalents at beginning of period	222,170	221,348	159,404
Cash and cash equivalents at end of period	70,519	222,170	221,348
Supplemental schedule of noncash investing and financing information
Issuance of common stock and exchange of stock options for business acquisition		64,845
Fixed assets in accounts payable and current liabilities	$ 115	$ 180	$ 178

Description of the Business

Description of the Business	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Description of the business	1. Description of the business Nature of Business —Adverum Biotechnologies, Inc. (the “Company” or “Adverum”) was incorporated in Delaware on July 17, 2006 as Avalanche Biotechnologies, Inc. and changed its name to Adverum Biotechnologies, Inc. on May 11, 2016. The Company is headquartered in Menlo Park, California. The Company is a clinical-stage gene therapy company targeting unmet medical needs in serious rare and ocular diseases. Leveraging a next-generation adeno-associated virus (“AAV”)-based directed evolution platform, the Company generates gene therapy product candidates designed to provide durable efficacy by inducing sustained expression of a therapeutic protein. The Company’s core capabilities include clinical development and in-house manufacturing expertise, specifically in process development, assay development, and current Good Manufacturing Practices (“cGMP”) quality control. Adverum is advancing a robust pipeline of gene therapy product candidates designed to treat rare diseases alpha-1 antitrypsin (“A1AT”) deficiency and hereditary angioedema (“HAE”), as well as wet age-related macular degeneration (“wAMD”). Since the Company’s inception, it has devoted its efforts principally to performing research and development activities, including conducting preclinical studies, early clinical trials, filing patent applications, obtaining regulatory agreements, hiring personnel, and raising capital to support these activities. The Company has not generated any revenue from the sale of products since its inception. The Company has experienced net losses since its inception and had an accumulated deficit of $254.1 million as of December 31, 2017. The Company expects to incur losses and have negative net cash flows from operating activities as it engages in further research and development activities. The Company believes that it has sufficient funds to continue operations through the end of 2019. In May 2016, the Company completed the acquisition of all the outstanding shares of Annapurna Therapeutics SAS (“Annapurna”), a privately-held French gene therapy company, in accordance with the terms of the acquisition agreement (the “Annapurna acquisition”) dated as of January 29, 2016, as amended on April 6, 2016. As a result, Annapurna is now a wholly owned subsidiary of the Company. Refer to Note 3 for more details. Upon completion of the Annapurna acquisition, the Company changed its name to “Adverum Biotechnologies, Inc.” The Company’s shares of common stock listed on The Nasdaq Global Market, previously trading through the close of business on Wednesday, May 11, 2016 under the ticker symbol “AAVL,” commenced trading on The Nasdaq Global Market under the ticker symbol “ADVM” on Thursday, May 12, 2016. Initial Public and Follow-on Offerings —In August 2014 and January 2015, the Company completed its initial public offering (IPO) and concurrent private placement and a follow-on offering and raised a total of net proceeds of $237.1 million. In March 2015, (i) the Company received net proceeds of approximately $8.3 million, after discounts and other issuance costs, which resulted from the sale of 230,000 shares of its common stock, and (ii) the Company issued 230,000 shares of its common stock to a stockholder that exercised warrants prior to the IPO. In August 2017, the Company entered into an at-the-market sales agreement with an agent for the sales of its common stock at market price (the “2017 stock offering agreement”). Under the terms and conditions of the 2017 stock offering agreement, the Company may offer to sell its common stock for an aggregate offering price of up to $50.0 million through the agent from time to time. During the year ended December 31, 2017, the Company issued and sold a total of 5,130,339 shares of its common stock at market prices under the 2017 stock offering agreement and raised total net proceeds of $16.5 million, net of issuance costs. In January 2018, the Company issued and sold a total of 1,419,893 shares of its common stock at market prices under the 2017 stock offering agreement and raised total net proceeds of approximately $5.7 million, net of issuance costs (see Note 17). In February 2018, the Company completed an underwritten public offering for the sale of 10,222,235 shares of its common stock and raised total net proceeds of approximately $64.3 million, after discounts and other issuance costs (see Note 17).

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of significant accounting policies Basis of Presentation and Principles of Consolidation — The Company’s consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”). The consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All inter-company transactions and balances have been eliminated in consolidation. Use of Estimates —The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, and the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of income and expenses during the reporting period. The Company bases estimates and assumptions on historical experience when available and on various factors that it believes to be reasonable under the circumstances. On an ongoing basis, the Company evaluates its estimates and assumptions, including those related to research and development expense accruals, stock-based compensation expense, income taxes, intangible asset, fair values of financial instruments and fair value of common stock warrants. The Company’s actual results may differ from these estimates under different assumptions or conditions. There have been no significant changes from the Company’s original estimates in any periods presented. Foreign Currency Translation —The Company’s consolidated financial statements are prepared in U.S. dollars. The Company’s foreign subsidiaries use the Euro and Australian dollar as their functional currencies and maintain their records in their local currencies, except its Ireland subsidiary that uses the U.S. dollar as its functional currency. Assets and liabilities are re-measured at exchange rates in effect at the end of the reporting period for the Company’s French and Australian subsidiaries, and at historical exchange rates for its Irish subsidiary. Equity is measured at historical rates and income and expenses are re-measured at average exchange rates for the reporting period. The resulting foreign currency translation adjustment is recorded in accumulated other comprehensive loss in the consolidated balance sheet. Transactions denominated in foreign currency are translated at exchange rates at the date of transaction with foreign currency gains (losses) recorded in other income (expense), net in the consolidated statements of operations and comprehensive loss. Cash and Cash Equivalents —The Company considers all highly liquid investments purchased with original maturities of three months or less at the date of purchase to be cash equivalents. Cash and cash equivalents include cash held in banks, money market accounts and highly liquid debt securities. Cash equivalents are stated at fair value. Short-Term Investments —All short-term investments, which consist of debt securities and certificates of deposit, have been classified as “available for sale” and are carried at fair value. Unrealized gains and losses, net of any related tax effects, are excluded from earnings and are included in other comprehensive loss and reported as a separate component of stockholders’ equity until realized. Realized gains and losses and declines in value judged to be other than temporary, if any, on available-for-sale securities are included in other income (expense), net in the Company’s consolidated statements of operations and comprehensive loss. The cost of securities sold is based on the specific-identification method. The amortized cost of securities is adjusted for amortization of premiums and accretion of discounts to maturity. Interest on short-term investments is included in other income, net in the Company’s consolidated statements of operations and comprehensive loss. In accordance with the Company’s investment policy, management invests to diversify credit risk and only invests in securities with high credit quality, including U.S. government securities. The Company periodically evaluates whether declines in the fair value of its investments below their cost are other than temporary. The evaluation includes consideration of the cause of the impairment, including the creditworthiness of the security issuers, the number of securities in an unrealized loss position, the severity and duration of the unrealized losses, whether the Company has the intent to sell the securities, and whether it is more likely than not that the Company will be required to sell the securities before the recovery of their amortized cost basis. If the Company determines that the decline in fair value of an investment is below its accounting basis and this decline is other than temporary, the Company would reduce the carrying value of the security it holds and records a loss for the amount of such decline. The Company has not recorded any realized losses or declines in value judged to be other than temporary on its investments. Segment Reporting —The Company operates and manages its business as one reporting and operating segment, which is the business of developing and commercializing gene therapeutics. The Company’s chief executive officer, who is the chief operating decision maker, reviews financial information on an aggregate basis for purposes of allocating resources and evaluating financial performance. Concentrations of Credit Risk and Other Uncertainties —Financial instruments that potentially subject the Company to significant concentrations of credit risk consists primarily of cash, cash equivalents and short-term investments. Risks associated with cash, cash equivalents, short-term investments are mitigated by the Company’s investment policy, which limits the Company’s investing to only those marketable securities rated at least A-1/P-1 Short Term Rating and A/A2 Long Term Rating, as determined by independent credit rating agencies. Management believes that the Company is not exposed to significant credit risk. The Company is subject to certain risks and uncertainties, including, but not limited to changes in any of the following areas that the Company believes could have a material adverse effect on future financial position or results of operations: ability to obtain future financing; regulatory approval and market acceptance of, and reimbursement for, the Company’s product candidates; performance of third-party clinical research organizations and manufacturers; development of sales channels; protection of the intellectual property; litigation or claims against the Company based on intellectual property, patent, product, regulatory or other factors; and the Company’s ability to attract and retain employees necessary to support the growth. Property and Equipment —Property and equipment are recorded at cost, net of accumulated depreciation and amortization. Depreciation is recorded using the straight-line method over the estimated useful lives of the assets, generally three to five years. Leasehold improvements are capitalized and amortized over the shorter period of their expected lives or the lease term. Major replacements and improvements are capitalized, while general repairs and maintenance are expensed as incurred. Valuation of Long-Lived Assets and Purchased Intangible Assets —The Company evaluates the carrying value of amortizable long‑lived assets, whenever events, or changes in business circumstances or the planned use of long‑lived assets indicate that their carrying amounts may not be fully recoverable or that their useful lives are no longer appropriate. If these facts and circumstances exist, the Company assesses for recovery by comparing the carrying values of long‑lived assets with their future undiscounted net cash flows. If the comparison indicates that impairment exists, long‑lived assets are written down to their respective fair values based on discounted cash flows. Significant management judgment is required in the forecasting of future operating results that is used in the preparation of expected undiscounted cash flows. If management’s assumptions about future operating results were to change as a result of events or circumstances, the Company may be required to record an impairment loss on these assets. No impairment indicators were noted for the Company’s amortizable long-lived assets, fixed assets, in the periods presented. The Company also evaluates the carrying value of intangible asset (not subject to amortization) related to in‑process research and development (“IPR&D”) asset, which is considered to be indefinite‑lived until the completion or abandonment of the associated research and development efforts. Accordingly, amortization of the IPR&D assets will not occur until the product reaches commercialization. During the period the intangible asset is considered indefinite‑lived, it will be tested for impairment on an annual basis, as well as between annual tests if the Company becomes aware of any events occurring or changes in circumstances that would indicate that the fair value of the IPR&D asset is less than its carrying amount. If and when development is complete, which generally occurs when regulatory approval to market the product is obtained, the associated IPR&D asset would be deemed definite‑lived and would then be amortized based on its estimated useful life at that point in time based on respective patent term. If a potential impairment exists, an impairment loss is measured as the excess of the asset’s carrying value over its fair value. During the year ended December 31, 2016, the Company recorded an impairment charge of $11.2 million related to its intangible assets (see Note 3). Financial Liabilities — During the year ended December 31, 2016, the Company entered into a sponsored research agreement with The Alpha-1 Project, Inc. (the “TAP”) with an embedded derivative, the Company determined to account for this financial liability at fair value and recorded as other non-current liabilities in its consolidated balance sheets (see Note 6). Change in fair value is recorded within other income, net in the Company’s consolidated statement of operations and comprehensive loss. Revenue Recognition —The Company has primarily generated revenue through license, research and collaboration arrangements with its strategic partners. The terms of these types of agreements may include (i) licenses to Adverum technology, (ii) research In arrangements involving the delivery of more than one element, each required deliverable is The arrangement’s consideration that is fixed or determinable is allocated to each separate unit Payments or reimbursements for the Company’s research and development efforts for the arrangements where The Company’s collaboration and license agreements may include contingent payments related to Collaboration and License Revenue Editas Agreement —In August 2016, the Company entered into a collaboration, option and license agreement with Editas Medicine, Inc. (“Editas”) (see Note 7). Under the terms of the agreement, the Company received initial payments of $1.0 million that included $0.5 million for research services during the year ended December 31, 2016. As the agreement provides for multiple deliverables, the Company accounts for this agreement as a multiple elements revenue arrangement. At the inception of the agreement, identified deliverables include research services, manufacturing of viral vectors for research, participation in joint research committee and exclusivity during the option period. These deliverables did not appear to have a standalone value and were combined into one unit of accounting. Options for each indication to license the Company’s AAV vector are considered substantive options and do not include significant incremental discounts. Therefore, they are not considered as deliverables under the agreement. In January 2018, the Company and Editas extended the research collaboration, option and license agreement (see Note 7). In consideration for extending the agreement, Editas made a one-time payment, non-refundable cash payment of $0.5 million to the Company in February 2018. The Company allocated the $1.0 million received to a single unit of accounting identified in the arrangement. The Company expects to recognize $1.0 million ratably over the associated period of performance, which is the maximum research period of three years. As there is no discernible pattern of performance and/or objectively measurable performance measures do not exist, the Company recognizes revenue on a straight-line basis. During the years ended December 31, 2017 and 2016, the Company recognized $0.3 million and $0.1 million, respectively, as collaboration and license revenue related to Editas agreement. Regeneron Agreement —In May 2014, the Company entered into a research collaboration and license agreement with Regeneron Pharmaceuticals, Inc. (“Regeneron”) to discover, develop and commercialize novel gene therapy products for the treatment of ophthalmologic diseases (see Note 7). Under the terms of the agreement, during the year ended December 31, 2014, the Company received initial upfront non-refundable cash payments of $8.0 million that included payment for research license fees, prepaid collaboration research costs and the time-limited right of first negotiation for license to develop and commercialize AVA-101. As the agreement provides for multiple deliverables, the Company accounts for this agreement as a multiple elements revenue arrangement. If deliverables did not appear to have a standalone value, they were combined with other deliverables into a unit of accounting with a standalone value. The Company allocated the $8.0 million to the relative fair value of the two units of accounting identified in the arrangement. The Company recognizes $6.5 million allocated to the first unit of accounting for the research licenses and related research and development services ratably over the associated period of performance, which is the maximum research period of eight years. As there was no discernible pattern of performance and/or objectively measurable performance measures did not exist for the first unit of accounting, the Company recognizes revenue on a straight-line basis over the eight years performance period. The remaining $1.5 million allocated to the second unit of accounting for the time-limited right of first negotiation for license to develop and commercialize AVA-101 was deferred. On November 2, 2015, Regeneron notified the Company that it did not exercise this right of first negotiation and the Company recognized the entire $1.5 million as revenue during the year ended December 31, 2015. As original research budget was fully used in the fourth quarter of 2015, the Company and Regeneron will agree on the reimbursement of additional research expenses annually. The Company invoices for services performed quarterly. These additional research fees are added to the research licenses and related research and development services unit of accounting, recorded as deferred revenue and recognized to revenue over the remaining research term. During the years ended December 31, 2017, 2016 and 2015, the Company recognized $1.5 million, $1.4 million and $0.8 million, respectively, related to the research licenses and research and development services unit of accounting in the Regeneron agreement. Research and Development Expenses —Research and development expenses are charged to expense as incurred. Research and development expenses include primarily personnel-related costs, stock-based compensation expense, laboratory supplies, consulting costs, external contract research and development expenses, including expenses incurred under agreements with CROs, the cost of acquiring, developing and manufacturing clinical study materials, and overhead expenses, such as rent, equipment depreciation, insurance and utilities. Advance payments for goods or services for future research and development activities are deferred and expensed as the goods are delivered or the related services are performed. The Company estimates preclinical studies and clinical trial expenses based on the services performed pursuant to contracts with research institutions and clinical research organizations that conduct and manage preclinical studies and clinical trials on the Company’s behalf. In accruing service fees, the Company estimates the time period over which services will be performed and the level of effort to be expended in each period. These estimates are based on communications with the third-party service providers and the Company’s estimates of accrued expenses and on information available at each balance sheet date. If the actual timing of the performance of services or the level of effort varies from the estimate, the Company will adjust the accrual accordingly. There have been no significant changes from the Company’s original estimates in any of the periods presented. The Company received tax credits from the Australian and French governments in connection with certain research costs incurred in conducting research by the Company’s Australian and French subsidiaries. These refunds do not depend on the taxable income or tax position of the Company and therefore the Company does not account for them under an income tax accounting model. The Company recognizes such tax credits in the period when qualified expenses are incurred as a reduction of research expenses. The Company has recorded the reimbursement of $0.1 million, $0.3 million and $0.1 million from the tax authorities as a reduction of research and development expense in the consolidated statements of operations and comprehensive loss in the years ended December 31, 2017, 2016 and 2015, respectively. Fair Value Measurements —Fair value is defined as 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. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. The carrying amounts of the Company’s financial instruments, including cash equivalents, prepaid and other current assets, accounts payable, accrued expenses and other current liabilities approximate their fair values due to their short-term maturities. Refer to Note 6 for the methodologies and assumptions used in valuing financial instruments. Stock-Based Compensation Expense —Stock-based compensation expense related to stock awards to employees is measured at fair value of the award on the date of the grant. The Company estimates the grant-date fair value, and the resulting stock-based compensation expense, using the Black-Scholes valuation model for stock options and using intrinsic value, which is the closing price of its common stock on the date of the grant, for the restricted stock units (“RSUs”). The fair value of the award that is ultimately expected to vest is recognized as expense on a straight-line basis over the requisite service period, which is generally the vesting period. As of January 1, 2017, the Company adopted Accounting Standard Update (“ASU”) No. 2016-09 and elected to account for forfeitures as they occur using a modified retrospective transition method, which requires the Company to record cumulative-effect adjustment to accumulated deficit. The Company determined that the impact of this adoption was immaterial and no adjustments were recorded in its consolidated financial statements. Prior to the adoption of ASU No. 2016-09, stock-based compensation expense recognized for the portion of the award that is expected to vest was reduced by an estimated forfeiture rate. The Black-Scholes valuation model requires the use of following assumptions: Expected Term —The expected term assumption represents the period that the Company’s stock-based awards are expected to be outstanding and is determined using the simplified method. Expected Volatility —Expected volatility is estimated using comparable public companies’ volatility for similar terms. Expected Dividend —The Black-Scholes valuation model calls for a single expected dividend yield as an input. The Company has never paid dividends and has no plans to pay dividends. Risk-Free Interest Rate —The risk-free interest rate is based on the U.S. Treasury zero-coupon issues in effect at the time of grant for periods corresponding with the expected term of option. Stock-based compensation expense related to awards granted to non-employees is recognized based on the then-current fair value at each measurement date over the associated service period of the award, which is generally the vesting term, using the accelerated attribution method. The fair value of non-employee stock options is estimated using the Black-Scholes valuation model with assumptions generally consistent with those used for employee stock options, with the exception of the expected term, which is the remaining contractual life at each measurement date. Refer to Note 13 for more information on assumptions used in estimating stock-based compensation expense. Income Taxes —The Company accounts for income taxes using the asset and liability method. The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. Deferred tax assets and liabilities are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. In evaluating the ability to recover its deferred income tax assets, the Company considers all available positive and negative evidence, including its operating results, ongoing tax planning and forecasts of future taxable income on a jurisdiction-by-jurisdiction basis. In the event the Company determines that it would be able to realize its deferred income tax assets in the future in excess of their net recorded amount, it would make an adjustment to the valuation allowance that would reduce the provision for income taxes. Conversely, in the event that all or part of the net deferred tax assets are determined not to be realizable in the future, an adjustment to the valuation allowance would be charged to earnings in the period when such determination is made. As of December 31, 2017 and 2016, the Company has recorded a full valuation allowance on its deferred tax assets. Tax benefits related to uncertain tax positions are recognized when it is more likely than not that a tax position will be sustained during an audit. Interest and penalties related to unrecognized tax benefits are included within the provision for income tax. Comprehensive Loss —Comprehensive loss is comprised of net loss and other comprehensive income (loss). Other comprehensive income (loss) consists of foreign currency translation adjustments related to translation of the financial statements of the Company’s Australia and France subsidiaries and unrealized gain (loss) on marketable securities. The Company did not have reclassifications from other comprehensive income (loss) to the income (loss) during the years ended December 31, 2017, 2016 and 2015. Basic and Diluted Net Loss Per Share — Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding for the period. Diluted net loss per share is computed by giving effect to all potential dilutive common stock equivalents outstanding for the period using the treasury stock method. Outstanding stock options, RSUs, ESPP and warrants are considered to be common stock equivalents and are only included in the calculation of diluted net loss per share when their effect is dilutive. Recent Accounting Standard Update Not Yet Effective — In May 2014, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606), which supersedes the revenue recognition requirements in ASC 605 , Revenue Recognition . This ASU is based on the principle that revenue is recognized to depict the transfer of goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. ASU 2014-09 defines a five-step process to achieve this core principle and, in doing so, it is possible more judgment and estimates may be required within the revenue recognition process than required under existing U.S. GAAP including identifying performance obligations in the contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. ASU 2014-09 is required to be adopted, using either of two methods: (i) retrospective to each prior reporting period presented with the option to elect certain practical expedients as defined within ASU 2014-09; or (ii) retrospective with the cumulative effect of initially applying ASU 2014-09 recognized at the date of initial application and providing certain additional disclosures. In July 2015, the FASB voted to approve a one-year deferral of the effective date to December 15, 2017 for interim and annual reporting periods beginning after that date and permitted early adoption of the standard, but not before the original effective date of December 15, 2016 . The FASB issued supplemental adoption guidance and clarification to ASU 2014-09 in March 2016, April 2016, May 2016, December 2016 and November 2017 within ASU 2016-08 Revenue From Contracts With Customers: Principal vs. Agent Considerations , ASU 2016-10 Revenue From Contracts with Customers: Identifying Performance Obligations and Licensing , ASU 2016-12 Revenue from Contracts with Customers: Narrow-Scope Improvements and Practical Expedients , ASU 2016-20 Technical Corrections and Improvement to Topic 606 – Revenue from Contracts with Customers and ASU 2017-14 Reporting Comprehensive Income (Topic 220), Revenue Recognition (Topic 605), and Revenue from Contracts with Customers (Topic 606), respectively. The Company will adopt the new standard in the first quarter of 2018 using the retrospective approach noted in (ii) above. The Company concluded that its collaboration agreements with Regeneron and Editas will be impacted by the adoption of the new revenue standards. The Company is in the process of allocating the transaction price to the performance obligations in each of the contracts. Preliminarily, the Company anticipates a material impact to its accounting policies, business processes, internal controls and disclosures . In January 2016, the FASB issued ASU No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities In February 2016, the FASB issued ASU No. 2016-2, Leases In June 2016, the FASB issued ASU No. 2016-13 Measurement of Credit Losses on Financial Instruments In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash In May 2017, the FASB issued ASU No. 2017-09, Scope of Modification Accounting The Company has reviewed other recent accounting pronouncements and concluded they are either not applicable to the business or no material effect is expected on the consolidated financial statements as a result of future adoption.

Acquisition of Annapurna

Acquisition of Annapurna	12 Months Ended
	Dec. 31, 2017
Business Combinations [Abstract]
Acquisition of Annapurna	3. Acquisition of Annapurna (a) Purchase Price Allocation On May 11, 2016, the Company completed the acquisition of all outstanding equity interests of Annapurna. Annapurna was a privately held French limited liability company and has two wholly-owned subsidiaries, Annapurna, Inc. in the U.S. and Annapurna Therapeutics Limited in Ireland. Annapurna was a biopharmaceutical company focused on discovering and developing novel gene therapy products for people living with severe rare diseases. The primary reasons for the acquisition were to expand the technology platforms within the Company’s research and development portfolio and to apply the Company’s resources and expertise in gene vectors development to advance Annapurna’s programs through development and clinical trials. Annapurna’s results of operations and fair value of assets acquired and liabilities assumed are included in the Company’s consolidated financial statements from the date of acquisition. The purchase price consideration was approximately $64.8 million based on the Company’s common stock closing price on Nasdaq on the acquisition closing date of $4.14 per share. A total of 14,087,246 shares of the Company’s common stock were issued to shareholders of Annapurna in exchange for all common and preferred stock outstanding at the closing date. Annapurna stockholders did not receive any fractional shares of the Company’s common stock in connection with the acquisition. Instead of receiving any fractional shares, each Annapurna stockholder was paid an amount in cash (without interest) equal to such fraction amount multiplied by the average 10 business days sale price of the Company’s common stock on Nasdaq from the acquisition date. Annapurna Series O preferred shares issued to founders were canceled prior to the acquisition date and were not included in the purchase price consideration. Vesting of certain of Annapurna’s stock options and unvested common shares was accelerated at the closing date. The fair value of stock awards related to the accelerated vesting of stock options and common shares of $0.9 million was excluded from the purchase price consideration and included in the Company’s operating expenses post acquisition. A portion of the purchase price was attributed to the exchange of Annapurna’s stock options and other rights to purchase capital stock outstanding at the acquisition closing date for corresponding common stock options of the Company at an exchange ratio of 9.54655. The Company reserved 3,673,940 shares for the future exercise of the Annapurna stock awards assumed in the acquisition. The total fair value of assumed Annapurna stock awards was approximately $14.7 million on the acquisition date, using the Black-Scholes option pricing model, assuming no dividends, expected volatilities of 80% and 89%, risk-free interest rates of 1.4% and 1.1%, and expected lives of six and ten years for employee and non-employee stock awards, respectively. Of the total fair value, $7.4 million was attributed as pre-acquisition service and included as part of the total purchase price consideration. The post-combination attribution amount of $7.2 million is recognized as compensation expense over the remaining requisite service period. The Company included $0.9 million in stock-based compensation expense related to the day-one post combination compensation expenses related to the accelerated vesting of stock options during the second quarter of 2016. Total purchase price consideration was estimated as follows (in thousands): Fair value of common shares issued $ 58,321 Fair value of the Company's common share options exchanged for Annapurna stock options and other stock awards attributable to pre-combination services 7,422 Less: value of common stock and accelerated vesting of stock options at close date (898 ) Total purchase price consideration $ 64,845 The transaction was accounted for using the acquisition method under ASC 805, Business Combinations, The allocation of total purchase price consideration is as follows (in thousands): Cash $ 3,449 Prepaid expenses and other assets 865 Property and equipment 185 Acquired intangible assets 16,200 Goodwill 49,514 Accounts payable (1,118 ) Accrued liabilities (1,848 ) Other noncurrent liabilities (377 ) Deferred tax liabilities (2,025 ) Total purchase price allocation $ 64,845 The identifiable intangible assets acquired consist of IPR&D assets related to products in development, as summarized in the table below (in thousands): Alpha-1 antitrypsin deficiency ("A1AT"),or ADVM-043 $ 11,700 Hereditary angioedema ("HAE"), or ADVM-053 4,500 Total acquired IPR&D intangible assets $ 16,200 The fair value of each IPR&D asset was estimated using the income approach and calculated using cash flow projections adjusted for inherent risks regarding regulatory approval, promotion, and distribution, discounted at a rate of approximately 11.0%. The Company acquired two additional intangible assets relating to the Friedreich’s Ataxia (“FA”) and severe allergy programs, but the fair value of each of these assets was determined to be nominal and was not included in the total acquired intangible assets. All IPR&D intangible assets acquired are classified as indefinite-lived and are not currently being amortized. Acquired IPR&D assets were impaired in the fourth quarter 2016 (see below discussion). Goodwill, which represents the excess of the purchase price over the fair values assigned to the net assets acquired, was $49.5 million on the acquisition date. The full amount of the goodwill was assigned to the entire Company, since management determined that the Company has only one reporting unit. The goodwill is not deductible for tax purposes. During the year ended December 31, 2016, the Company’s goodwill was fully impaired (see below discussion). The amount of net loss of Annapurna included in the consolidated statements of operations and comprehensive loss from the acquisition date through the period ended December 31, 2016 was $16.6 million, which included $11.2 million of impairment charges related to IPR&D assets. Annapurna did not generate any revenues prior or post acquisition. The pro forma financial information combines the results of operations of Adverum and Annapurna as though the businesses had been combined as of the beginning of fiscal 2015. The pro forma financial information is presented for informational purposes only, and is not indicative of the results of operations that would have been achieved in the current or any future periods. The following table presents the unaudited pro forma combined results of operations (in thousands, except per share data). Years ended December 31, 2016 2015 Pro forma information Collaboration and license revenue $ 1,455 $ 2,319 Net loss $ (117,551 ) $ (61,774 ) Basic and diluted loss per share $ (2.85 ) $ (1.56 ) Weighted-average common shares outstanding - basic and diluted 41,288 39,566 Pro-forma adjustments included the following: • Actual acquisition-related transaction costs of $2.5 million for year ended December 2016 were excluded from the 2016 pro forma results above. As these expenses were incurred prior to the closing of the acquisition, they were not included in the 2015 pro forma results. • $0.9 million of stock-based compensation expense related to the accelerated vesting associated with the Annapurna acquisition was excluded from the 2016 pro forma results and was, instead, included in the 2015 pro forma results. • Stock-based compensation expense of $0.2 million and $0.4 million related to stock options granted to executives upon the closing of the Annapurna acquisition was included in the 2016 and 2015 pro forma results, respectively. • Interest expense related to convertible notes and changes in fair value of preferred stock warrants of $0.5 million for the year ended December 31, 2015 and $1.0 million for the year ended December 31, 2016, were excluded from 2015 and 2016 pro-forma results above, as the convertible notes and warrants were settled prior to the closing of the Annapurna acquisition. • Bonuses of $0.4 million paid in connection with closing of the Annapurna acquisition were excluded from the 2016 pro forma results and were, instead, included in the 2015 pro forma results. The unaudited condensed pro forma information does not include any anticipated synergies that may be achievable subsequent to the date of acquisition. b) Impairment Evaluation for Goodwill and Intangible Assets As the Company recorded goodwill and IPR&D intangible assets upon the Annapurna acquisition, the Company is required to test goodwill and indefinite lived intangible assets for impairment on an annual basis or more frequently if indicators of impairment exist. The Company operates as one reporting unit and goodwill was allocated to this reporting unit. During the second quarter of 2016, the Company noted a continuing decrease in its stock price that resulted in the market capitalization being less than the carrying value of the Company’s net assets as of June 30, 2016. As the operating losses were expected to increase significantly in the following years due to continuing pre-clinical and expected clinical trials, the Company concluded that it was more likely than not that the fair value of the Company’s one reporting unit was less than its carrying value and, as a result, performed a step one goodwill impairment analysis. In performing the step one analysis, the Company determined the fair value of the reporting unit using a market-based approach. The Company multiplied the stock price of $3.16 by the 41.3 million of its common shares outstanding on June 30, 2016 and applied a control premium to estimate the common equity value on a controlling basis. As the fair value was less than the carrying value of the Company’s net assets, the Company proceeded to step two of the impairment analysis. The second step of the analysis includes allocating the calculated fair value (determined in the step one analysis) of the reporting unit to its assets and liabilities to determine an implied fair value of goodwill. The implied fair value of goodwill was determined in the same manner as the amount of goodwill recognized in an acquisition. That is, the estimated fair value of the reporting unit was allocated to all of the assets and liabilities as if the Company had been acquired and the estimated fair value was the purchase price paid. As part of this assessment the Company considered the preliminary valuation of Annapurna net assets acquired, excluding goodwill, as their fair value from May 11, 2016, the acquisition closing date, to June 30, 2016 did not change. The Company also noted that the fair value of current assets and liabilities approximated their carrying value due to their short-term nature, the Company’s cash and cash equivalent balance was higher than the fair value estimated in the step one analysis, and the fair value of fixed assets approximated their recorded value as most of the Company’s fixed assets were acquired in the last couple of years. Based on this analysis, the implied fair value of the goodwill was zero. Accordingly, the Company fully impaired its goodwill and recorded an impairment charge was $49.5 million, which is included within impairment of goodwill and intangible assets in the Company’s consolidated statements of operations and comprehensive loss for the year ended December 31, 2016. In the fourth quarter of 2016, the Company performed its annual impairment assessment of its intangible assets, ADVM-043 and ADVM-053. Based on the Company’s decision in the fourth quarter to change its manufacturing process for ADVM-043 and ADVM-053 by implementing its proprietary baculovirus-based production system, it updated the related development and manufacturing costs. As a result, the Company revised its forecasts for manufacturing and related costs. In addition, the Company also reviewed and updated its expected timing of clinical trials, receipts of regulatory approvals and costs to complete. Based upon the Company’s analysis, it determined that the total carrying value of $16.2 million for its intangible assets, ADVM-043 and ADVM-053, was higher than their total fair value of $5.0 million. Accordingly, the Company recorded $11.2 million impairment charge. Intangible asset related to ADVM-053 was fully impaired and intangible assets related to ADVM-043 was impaired and had fair value of $5.0 million as of December 31, 2016. Impairment charges for goodwill and intangible assets are included in the Company’s consolidated statements of operations and comprehensive loss. In the fourth quarter of 2017, the Company performed its annual impairment assessment of its intangible asset, ADVM-043. Based on the Company’s impairment analysis, it concluded that its ADVM-043 IPR&D asset was not impaired.

Restructuring Charges

Restructuring Charges	12 Months Ended
	Dec. 31, 2017
Restructuring And Related Activities [Abstract]
Restructuring Charges	4. Restructuring Charges On December 22, 2015, the Company implemented a restructuring plan to reduce operating costs and better align its workforce with the needs of its business following its decision to not initiate the Phase 2b clinical trial for AVA-101 during the second half of 2015. The plan resulted in a reduction of approximately 20% of the Company’s workforce, or a total of 15 employees. Affected employees were eligible to receive severance payments. The plan also triggered accelerated vesting of certain of the affected employees’ restricted stock units (“RSUs”). In connection with the restructuring plan, the Company estimated aggregate restructuring charges of approximately $2.6 million, which were recorded in the Company’s consolidated statements of operations and comprehensive loss during the year ended December 31, 2015, related to one-time termination severance payments and other employee-related benefits, which included approximately $1.0 million of stock-based compensation expense related to the acceleration of RSUs. The following table summarizes the restructuring activities (in thousands): One-Time Termination Benefits Non-Cash Charge Related to Acceleration of RSUs Total Restructuring liability as of December 31, 2014 $ — $ — $ — Costs incurred and recorded as restructuring charges 1,524 1,049 2,573 Cash payments (511 ) — (511 ) Non-cash settlements — (1,049 ) (1,049 ) Restructuring liability as of December 31, 2015 1,013 — 1,013 Cash payments (988 ) — (988 ) Restructuring liability as of December 31, 2016 25 — 25 Cash payments (25 ) — (25 ) Restructuring liability as of December 31, 2017 $ — $ — $ —

Cash Equivalents and Short-Term

Cash Equivalents and Short-Term Investments	12 Months Ended
	Dec. 31, 2017
Cash And Cash Equivalents [Abstract]
Cash Equivalents and Short-Term Investments	5. Cash Equivalents and Short-Term Investments The following is a summary of the Company’s cash equivalents and short-term investments (in thousands): December 31, 2017 Amortized Cost Basis Unrealized Gains Unrealized Loses Estimated Fair Value Money market funds $ 65 $ — $ — $ 65 U.S. government and agency securities 58,351 — (145 ) 58,206 Commercial paper 71,427 — — 71,427 Corporate bonds 38,354 1 (38 ) 38,317 Certificates of deposit 9,731 — — 9,731 Total cash equivalents and short-term investments 177,928 1 (183 ) 177,746 Less: Cash equivalents (57,780 ) — — (57,780 ) Total short-term investments $ 120,148 $ 1 $ (183 ) $ 119,966 December 31, 2016 Money market funds - cash equivalent $ 215,916 $ — $ — $ 215,916 Total cash equivalents $ 215,916 $ — $ — $ 215,916 Management determined that the gross unrealized losses on the Company’s marketable securities as of December 31, 2017 were temporary in nature. Therefore, none of the Company’s marketable securities were other-than-temporarily impaired as of December 31, 2017. As of December 31, 2016, all of the Company’s investments in marketable securities were held in money market funds and were classified as cash equivalents. The following table is a summary of the cost and estimated fair value of the Company’s marketable securities based on stated effective maturities as of December 31, 2017: Amortized Estimated Cost Basis Fair Value (In thousands) Mature within one year $ 169,909 $ 169,758 Mature after one year to three years 8,019 7,988 Total cash equivalents and short-term investments $ 177,928 $ 177,746

Fair Value Measurements and Fai

Fair Value Measurements and Fair Value of Financial Instruments	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value Measurements and Fair Value of Financial Instruments	6. Fair Value Measurements and Fair Value of Financial Instruments The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows: Level 1: Quoted 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. Assets and liabilities measured at fair value are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. The Company’s assessment of the significance of a particular input to the fair value measurement in its entirety requires management to make judgments and consider factors specific to the asset or liability. The fair value of Level 1 securities is determined using quoted prices in active markets for identical assets. Level 1 securities consist of highly liquid money market funds. Financial assets and liabilities are considered Level 2 when their fair values are determined using inputs that are observable in the market or can be derived principally from or corroborated by observable market data such as pricing for similar securities, recently executed transactions, cash flow models with yield curves, and benchmark securities. In addition, Level 2 financial instruments are valued using comparisons to like-kind financial instruments and models that use readily observable market data as their basis. U.S. government and agency securities, commercial paper, corporate bond and certificates of deposit are valued primarily using market prices of comparable securities, bid/ask quotes, interest rate yields and prepayment spreads and are included in Level 2. In certain cases where there is limited activity or less transparency around inputs to valuation, securities are classified as Level 3 within the valuation hierarchy. Level 3 liabilities that were measured at estimated fair value on a recurring basis consist of a financing arrangement entered during the year ended December 31, 2016. During the periods presented, the Company has not changed the manner in which it values liabilities that are measured at estimated fair value using Level 3 inputs. There were no transfers within the hierarchy during the years ended December 31, 2017 and 2016. The following table summarizes, for assets and liabilities recorded at fair value, the respective fair value and the classification by level of input within the fair value hierarchy as described above (in thousands): Quoted Prices Significant Other Significant Total In Observable Inputs Unobservable Inputs Carrying (Level 1) (Level 2) (Level 3) December 31, 2017 Assets: Money market funds $ 65 $ 65 $ — $ — U.S. government and agency securities 58,206 — 58,206 — Commercial paper 71,427 — 71,427 — Corporate bonds 38,317 38,317 Certificates of deposit 9,731 — 9,731 — Total cash equivalents and short-term investments $ 177,746 $ 65 $ 177,681 $ — Other noncurrent liability: Financing arrangement $ 157 $ — $ — $ 157 December 31, 2016 Assets: Money market funds $ 215,916 $ 215,916 $ — $ — Total cash equivalents $ 215,916 $ 215,916 $ — $ — Other noncurrent liability: Financing arrangement $ 74 $ — $ — $ 74 In August 2016, the Company entered into a financing arrangement with the TAP for a total amount of up to $0.3 million (the “TAP financing”), of which approximately $0.2 million and $0.1 million, respectively, was outstanding as of December 31, 2017 and 2016 (see Note 10). The Company elected the fair value option to account for this financing arrangement. The fair value of the financing arrangement was determined based on the expected value approach and is classified as Level 3 within the fair value hierarchy. The Company determined that the changes in the fair value were immaterial during the years ended December 31, 2017 and 2016. The key unobservable inputs in the valuation model include timing of milestones, probability of achievement of development and commercial milestones, and a discount factor. The following table presents quantitative information about the inputs and valuation methodologies used for the fair value measurements classified in Level 3 of the fair value hierarchy: Weighted-Average (range - if applicable) Fair Value at December 31, Valuation Significant Unobservable as of December 31, 2017 2016 Methodology Input 2017 2016 (In thousands) TAP financing $ 157 $ 74 Expected value approach Milestone dates: 2018 - 2023 2017 - 2023 Discount rate: 5.8% 5.5% Percent probability of milestone achievements: 18.2% to 65.0% 18.2% to 80.0% Non-financial assets such as intangible assets, property, plant, and equipment are evaluated for impairment and adjusted to their fair value using Level 3 inputs, only when impairment is recognized. Fair values are considered Level 3 when management makes significant assumptions in developing a discounted cash flow model based upon a number of considerations, including projections of revenues, earnings and a discount rate. In addition, in evaluating the fair value of goodwill impairment, further corroboration is obtained using the Company’s market capitalization.

Significant Agreements

Significant Agreements	12 Months Ended
	Dec. 31, 2017
Research And Development [Abstract]
Significant Agreements	7. Significant Agreements University of California Agreement In May 2010, the Company entered into a license agreement with the Regents of University of California (the “Regents”), as amended in September 2013. Under the license agreement, the Regents granted to the Company an exclusive (even as to the Regents) license, with the right to grant sublicenses, under the Regents’ undivided interest in patent rights covering a method of using recombinant gene delivery vectors, including AAV7m8 in the Company’s wet AMD product candidate ADVM-022, for treating or preventing diseases of the eye, to develop, make, have made, use offer for sale, import, export and sell products covered by such patent rights in all fields of use in the U.S. The licensed patent rights are jointly owned by the Regents and Chiron Corporation, which was acquired by Novartis AG, but the Company’s license extends only to the Regents’ interest in such patent rights. The Company is obligated to make milestone payments totaling up to $0.9 million upon reaching certain stages of development of the licensed products for one indication, and totaling up to $0.5 million for each subsequent indication for which licensed products are developed, for up to a maximum of two additional indications. Through December 31, 2017, none of these goals had been achieved, and no milestones were payable. The Company’s license agreement with the Regents continues in effect for the life of the last-to-expire patent. The Company expects the agreement to terminate prior to any commercialization of any product candidates to which they apply. The Company may terminate this agreement without cause at any time upon 30 days’ prior written notice to the Regents. The Regents may terminate this agreement for a breach by the Company that remains uncured for 60 days, if the Company becomes insolvent, if the Company directly or through a third party files a claim that a licensed patent right is invalid or unenforceable, or if the Company fails to meet or extend the date for meeting certain diligence milestones. Regeneron Agreement In May 2014, the Company entered into a research collaboration and license agreement with Regeneron to discover, develop and commercialize novel gene therapy products for the treatment of ophthalmologic diseases. The collaboration covers up to eight distinct therapeutic targets (“collaboration targets”). The Company and Regeneron collaborated during the initial research period of three years. In February 2017, Regeneron exercised its option to extend the research collaboration and license agreement for an additional three years, through May 1, 2020. During the research period, Regeneron has the option to obtain an exclusive worldwide license for a collaboration target’s further development by giving written notice to the Company and paying $2.0 million per target. If Regeneron exercises its option, it will be responsible for all further development and commercialization of the target. The Company is then eligible to receive contingent payments of up to $80.0 million upon achievement of certain development and regulatory milestones for product candidates directed toward each collaboration target, for a combined total of up to $640.0 million in potential milestone payments for product candidates directed toward all eight collaboration targets, plus a royalty in the low- to mid-single-digits on worldwide net sales of collaboration products. For any two collaboration targets, the Company has an option to share up to 35% of the worldwide product candidate development costs and profits. If the Company exercises this option, the Company will not be eligible for milestone and royalty payments as discussed above but rather the Company will share development costs and profits with Regeneron. The agreement will expire with respect to each collaboration target upon expiration of all payment obligations by Regeneron. In addition, the agreement, or Regeneron’s rights to any target development under the agreement, may terminate early under the following situations: • Regeneron may terminate the agreement for convenience at any time on a target by target basis or in totality upon a 30-day notice. • Each party can terminate the agreement if another party commits a material breach or material default in performance of its obligations and such breach or default is not cured within 60 days. • The agreement is automatically terminated upon initiation of any bankruptcy proceedings, reorganization or dissolution of either party. • The Company can terminate the agreement upon 30-day notice if Regeneron challenges the validity, scope or enforceability of any Company patent. Under the Company’s research, collaboration and license agreement with Regeneron, the Company is required to have a mutually agreed-on research plan with Regeneron in order to invoice Regeneron for services performed. The Company does not currently have a research plan in place, and, consequently, it does not currently receive any reimbursements from Regeneron. Editas Agreement In August 2016, the Company entered into a collaboration, option and license agreement with Editas pursuant to which the Company and Editas collaborate on certain studies using AAV vectors in connection with Editas’ genome editing technology and the Company grants to Editas an exclusive option to obtain certain exclusive rights to use the Company’s proprietary vectors in up to five ophthalmic indications. The Company received a $1.0 million non-refundable upfront payment during the year ended December 31, 2016, with $0.5 million of such payment to be credited against Editas’ obligation to fund research and development costs. Under the terms of the agreement, both the Company and Editas are subject to exclusivity obligations. In January 2018, the Company and Editas extended the research collaboration, option and license agreement. In consideration for extending the agreement, Editas made a one-time, non-refundable cash payment of $0.5 million to the Company in February 2018 (see Note 17). Under the terms of the agreement, as amended, Editas may exercise the option with respect to a designated initial indication until September 30, 2018. With respect to the four other indications, Editas may exercise the option until the fourth anniversary of the effective date, provided that the option will expire on the third anniversary of the effective date if Editas has not exercised the option with respect to the initial indication or any other indication by such date. Upon Editas’ timely exercise of the option with respect to the designated initial indication, Editas will pay the Company a $1.3 million fee. For the first additional indication for which Editas timely exercises its option, Editas will pay the Company a $1.5 million fee. Upon each subsequent exercise of the option, Editas will pay the Company a $1.0 million fee per Indication. If Editas elects to develop a product using certain of the Company’s proprietary vectors, the Company will be eligible to receive up to $15.5 million in development and commercialization milestone payments for such product, and tiered royalties between the mid-single digits and low teens on net sales of such product, subject to certain adjustments. Unless earlier terminated, the agreement will be in effect until the later of the expiration of the option exercise period or the expiration of the royalty term of the last product. At any time after the option is first exercised, Editas may terminate the agreement for convenience in its entirety or on an indication-by indication or country-by-country basis, upon prior written notice to the Company. The Company may also terminate the agreement if Editas challenges the Company’s patents relating to its proprietary vectors and does not withdraw such challenge within a defined period of time. In addition, either party may terminate the agreement with written notice upon a bankruptcy of the other party or upon an uncured material breach by the other party. Cornell University Agreement The Company had been a party to a master service agreement (the “MSA”) with Cornell University (“Cornell”) originally established in August 2014 and amended in December 2015. Under the MSA, Cornell provided assistance in regulatory affairs, overall project management, and parameter development. Subsequent to the Annapurna acquisition, the Company recorded a total research and development expenses of $1.8 million related to Cornell agreements for the year ended December 31, 2016. In December 2016, the Company informed Cornell that the Company decided to terminate the MSA for material breach, effective January 6, 2017. Subsequently, Cornell informed the Company that it disputes the validity of the Company’s termination of the MSA. Although the Company intends to defend the validity of its termination of the MSA, the Company recorded $2.0 million of estimated costs associated with the termination of the MSA as of December 31, 2017. The MSA included services relating to the Company’s gene therapy programs ADVM-043, ADVM-053 and severe allergy. The Company’s three licensing agreements with Cornell for these programs remain unchanged. The decision to terminate the MSA was due to Cornell’s failure to deliver therapeutic material of ADVM-043 suitable for use in human patients. As a result of this decision, the Company contracted with large-scale contract manufacturing organizations that comply with current good manufacturing practice industry standards and can produce product quantities for both the Company’s planned clinical trials and potential commercial supply. This is part of the Company’s upgrade of the manufacturing process for ADVM-043. In December 2015, the Company entered into three licensing agreements with Cornell, pursuant to which the Company will advance its gene therapy programs ADVM-043, ADVM-053, and severe allergy, which originally were initiated at the Department of Genetic Medicine at Weill Cornell. A1AT Deficiency License Agreement — Under this agreement, the Company holds an exclusive license to certain know-how related to A1AT deficiency and rights to an Investigational New Drug (IND) application to initiate clinical studies of gene therapy for A1AT. HAE License Agreement — Under this agreement, the Company holds an exclusive license to certain technology related to hereditary angioedema (HAE) and a non-exclusive license to certain other intellectual property related to the HAE program. Allergy License Agreement — Under this agreement, the Company holds an exclusive license to certain patents related to allergens and a non-exclusive license to certain other technology related to allergens. Across these three license agreements, Cornell is entitled to receive aggregate annual maintenance fees ranging from $30,000 to $0.3 million per year, up to $16.0 million in aggregate milestone payments and royalties on sales in the low single-digits, subject to adjustments and minimum thresholds. For the year ended December 31, 2017 and 2016, annual maintenance fees were immaterial. No milestone payments were probable to achieve and none were recorded as of December 31, 2017 and 2016. Dr. Crystal, Chairman of Genetic Medicine, the Bruce Webster Professor of Internal Medicine and a Professor of Genetic Medicine and of Medicine at Weill Cornell, served as a consultant to the Company since the inception of Annapurna and continues to provide services for annual base compensation of $0.2 million. REGENXBIO Agreement A1AT Deficiency/Allergy License Agreement — In October 2015, the Company entered into an exclusive worldwide license to certain intellectual property in order to make, have made, use, import, sell and offer for sale certain licensed products for the treatment of A1AT deficiency. Under this agreement, the Company has an option to be granted an exclusive worldwide license to certain intellectual property related to the treatment of severe allergies, which option expired in October 2016. Also, under this license agreement, REGENXBIO, Inc. (“REGENXBIO”) is eligible to receive annual maintenance fees, up to approximately $20.0 million in combined milestone payments and royalties in the mid-to-high single digits. In April 2017, the Company exercised its right to terminate the license agreement for any reason upon six months’ prior written notice. The termination was effective in October 2017. FA License Agreement — In April 2014, the Company entered into an exclusive worldwide license to certain intellectual property related to the FA program to make, have made, use, import, sell and offer for sale licensed products using AAVrh10 for FA where the vector is administered by any route except directly to the central nervous system (“FA Systemic”). Under the terms of this license agreement, the Company also had an option to obtain a non-exclusive worldwide license to make, have made, use, import, sell and offer for sale licensed products using a single vector for each of FA where the vector is administered directly to the central nervous system (“FA CNS”) and FA Systemic. Under this license agreement, REGENXBIO is eligible to receive annual maintenance fees, up to $13.9 million in combined milestone fees and royalties in the mid-to-high single digits. The option to obtain a non-exclusive license to FA Systemic expired in April 2015 and the option to obtain a non-exclusive license for FA CNS expired in April 2016. The Company is obligated to achieve certain development milestones with respect to each licensed disease indication, including the filing of an IND application for each licensed disease indication within a specified time period, which it may extend for additional time for a specified number of extensions upon the payment of a fee. In October 2017, the Company notified REGENXBIO that the Company exercised its right to terminate this license agreement for any reason upon six months’ written notice. The termination will be effective in April 2018. Inserm Transfert In July 2014, the Company entered into an agreement with Inserm Transfert (“Inserm”) whereby it holds an exclusive license to certain patents to develop, make, have made, use, import, offer for sale and sell or otherwise distribute products for the treatment of FA and a non-exclusive license to certain other intellectual property related to the FA program. The agreement was amended in October 2015 to increase the scope of the intellectual property under the licenses. Under this agreement, Inserm is entitled to receive certain de minimis license payments, certain development milestone payments of up to approximately €2.0 million in the aggregate and royalties on sales in the low single-digits, subject to adjustments. No milestone payments were probable to achieve and none were recorded as of December 31, 2017. Unless earlier terminated, this agreement will be in effect on a country-by-country, licensed product-by-licensed product basis the last claim of the licensed intellectual property Inserm may terminate this license agreement if the Company becomes the subject of a voluntary or involuntary petition in bankruptcy or fails to meet development milestones and such failure is not cured within a specified number of days. Inserm may also terminate the license granted to the Company in a given country if the Company (i) before regulatory approval of a product in any country, has ceased conducting any development of products in all countries for 12 consecutive months or (ii) after regulatory approval of a product in a given country, has ceased marketing such product in such country for 12 consecutive months. Pursuant to the terms of the agreement with Inserm, due to the acquisition of Annapurna, the Company made a one-time payment of €0.3 million to Inserm during the year ended December 31, 2017.

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Property and Equipment, Net	8. Property and Equipment, Net Property and equipment, net consists of the following: December 31, 2017 2016 (In thousands) Computer equipment and software $ 535 $ 300 Laboratory equipment 4,956 4,285 Furniture and fixtures 552 552 Leasehold improvements 1,549 1,522 Construction in progress 105 104 Total property and equipment 7,697 6,763 Less accumulated depreciation and amortization (4,673 ) (2,594 ) Property and equipment, net $ 3,024 $ 4,169 Depreciation and amortization expense related to property and equipment was $2.1 million, $1.6 million and $0.8 million for the years ended December 31, 2017, 2016 and 2015, respectively.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	9. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consist of the following: December 31, 2017 2016 (In thousands) Employees' compensation $ 2,259 $ 2,570 Accrued preclinical costs 1,255 1,683 Accrued professional fees 2,295 894 Accrued clinical and process development costs 910 1,142 Other 245 187 Total accrued expenses and other current liabilities $ 6,964 $ 6,476

Financing Arrangements

Financing Arrangements	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Financing Arrangements	10. Financing Arrangements Banque Publique d’Investissement (“BPI France”) Agreement In August 2015, BPI France granted Annapurna a €0.8 million interest-free conditional advance, of which €0.5 million was outstanding as of December 31, 2016. Payments are scheduled in equal quarterly amounts of €25,000 from September 30, 2017 to June 30, 2022. This payment schedule will be modified if the Company will receive revenue from license or product sales before advances are paid in full. The Company calculated 7% imputed interest expense on these advances that was recorded as a discount at the issuance date. The discount is amortized as an interest expense over the life of the advances. As of December 31, 2017 and 2016, the total carrying value, which approximates the fair value, of the conditional advance was $0.4 million, of which $0.3 million was recorded within other non-current liabilities and $0.1 million within accrued expenses and other current liabilities in the Company’s consolidated balance sheets. The TAP Agreement In July 2016, the Company entered into a sponsored research agreement with The TAP in which the TAP will fund the Company’s A1AT research activities of up to $0.3 million in cash in three different tranches. The Company may repay up to 4.5 times the received amount if and when certain product approval and sales milestones are achieved. In September 2016, the Company received $0.1 million and issued warrant to purchase 10,000 shares of its common stock exercisable anytime during five years from the issuance date at an exercise price of $4.33 per share (the “TAP warrant). In December 2017, the Company achieved a milestone which entitled the Company to receive $0.1 million. For the valuation details of the TAP financing, refer to Note 6. The following table presents the TAP financing activity: Years ended December 31, 2017 2016 (In thousands) Balance of TAP financing liability as of the beginning of the year (1) $ 74 $ — Funding (2) 100 100 Fair value of common stock warrant issued (see Note 12) (3) — (26 ) Gain on fair value of TAP financing liability (4) (17 ) — Balance of TAP financing liability as of the end of the year $ 157 $ 74 ___________ (1) Recorded within other non-current liabilities in the Company’s consolidated balance sheets. (2) Recorded as a receivable as of December 31, 2017. Payment was received in January 2018. (3) Recorded within other non-current liabilities and additional paid-in capital in the Company’s consolidated balance sheets. (4) Recorded within other non-current liabilities in the Company’s consolidated balance sheet and other income, net in the Company consolidated statement of operations and comprehensive loss.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	11. Commitments and Contingencies Facility Lease Agreement The Company leases its’ office building under a non-cancelable lease agreement, which expires on May 8, 2020. The Company may extend this lease for up to four years. The lease agreement provides for an escalation of rent payments each year. The Company records rent expense on a straight-line basis over the term of the lease. As of December 31, 2017, future minimum commitments under the Company’s facility operating lease were as follows: Years ended December 31, Future Commitments (In thousands) 2018 $ 1,162 2019 1,197 2020 403 Total minimum lease payments $ 2,762 Rent expense recognized under the operating lease, including additional rent charges for utilities, parking, maintenance, and real estate taxes was $1.8 million, $1.7 million and $1.5 million for the years ended December 31, 2017, 2016 and 2015, respectively. Contractual Obligations As of December 31, 2017, the Company had a contractual obligation of approximately $4.2 million for a contract manufacturing with a vendor Collaborations and License Agreements The Company is a party to various agreements, principally relating to licensed technology that requires future payments relating to milestones or royalties on future sales of specified products. Through December 31, 2017, none of the goals had been achieved under the license agreements and no milestones were accrued or payable. Because the achievement of these milestones is not fixed and determinable, such commitments have not been included on the Company’s consolidated balance sheets. Aggregate annual maintenance fee’s payments were approximately $0.5 and $0.6 million for each of the years ended December 31, 2017 and 2016. Guarantees and Indemnifications In the normal course of business, the Company enters into contracts and agreements that contain a variety of representations and warranties and provide for indemnification for certain liabilities. The exposure under these agreements is unknown because it involves claims that may be made against the Company in the future but have not yet been made. To date, the Company has not paid any claims or been required to defend any action related to its indemnification obligations. However, the Company may record charges in the future as a result of these indemnification obligations. The Company also has indemnification obligations to its directors and executive officers for specified events or occurrences, subject to some limits, while they are serving at the Company’s request in such capacities. There have been no claims to date and the Company believes the fair value of these indemnification agreements is minimal. Accordingly, the Company has not recorded any liabilities for these agreements as of December 31, 2017 and 2016. Legal Proceedings From time to time, the Company may become involved in litigation and other legal actions. The Company estimates the range of liability related to any pending litigation where the amount and range of loss can be estimated. The Company records its best estimate of a loss when the loss is considered probable. Where a liability is probable and there is a range of estimated loss with no best estimate in the range, the Company records a charge equal to at least the minimum estimated liability for a loss contingency when both of the following conditions are met: (i) information available prior to issuance of the financial statements indicates that it is probable that a liability had been incurred at the date of the financial statements and (ii) the range of loss can be reasonably estimated. In July 2015, three securities class action lawsuits were filed against the Company and certain of its officers in the United States District Court for the Northern District of California (“U.S. District Court”), each on behalf of a purported class of persons and entities who purchased or otherwise acquired the Company’s publicly traded securities between July 31, 2014 and June 15, 2015. The lawsuits assert claims under the Securities Exchange Act of 1934, as amended (the “Exchange Act”) and Securities Act of 1933, as amended (the “Securities Act”) and allege that the defendants made materially false and misleading statements and omitted allegedly material information related to, among other things, the Phase 2a clinical trial for AVA-101, a product candidate which is no longer being developed, and the prospects of AVA-101. The complaints seek unspecified damages, attorneys’ fees and other costs. In December 2015, a putative securities class action lawsuit was filed against the Company, the Company’s board of directors, underwriters of the Company’s January 13, 2015, follow-on public stock offering, and two of the Company’s institutional stockholders, in the Superior Court of the State of California for the County of San Mateo (“San Mateo Superior Court”). The complaint alleges that, in connection with the Company’s follow-on stock offering, the defendants violated the Securities Act by allegedly making materially false and misleading statements and by allegedly omitting material information related to the Phase 2a clinical trial for AVA- 101 and the prospects of AVA-101. The complaint seeks unspecified compensatory and rescissory damages, attorneys’ fees and other costs. The plaintiff has dismissed the two institutional stockholder defendants. On March 16, 2017, the Company reached an agreement to settle the asserted actions. The proposed aggregate amount of the settlement is $13.0 million, of which $1.0 million would be contributed by the Company to cover its indemnification obligations to the underwriters, and the remainder would be contributed by the Company’s insurers. The Company and the defendants have denied and continue to deny each and all of the claims alleged in the actions, and the settlement does not assign or reflect any admission of fault, wrongdoing or liability as to any defendant. Notice of the settlement was provided to shareholders in the fall of 2017, and no shareholder objected to the settlement. On January, 19, 2018, the San Mateo Superior Court entered a judgment and order finally approving the settlement. And on February 5, 2018, the U.S. District Court entered an order dismissing the consolidated federal action with prejudice. If the settlement does not become effective and litigation resumes, following an appeal or otherwise, adverse outcomes in the actions could result in substantial damages. The Company recorded $1.0 million as general and administrative expense during the three months ended March 31, 2017, when the amount and time of settlement became estimable and probable.

Stockholders_ Equity

Stockholders’ Equity	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Stockholders’ Equity	12. Stockholders’ Equity Common Stock and Preferred Stock The Company’s authorized share capital is 300,000,000 shares of common stock and 5,000,000 shares of preferred stock. As of December 31, 2017 and 2016, the Company had no preferred stock issued and outstanding. Common Stock Warrants The Lions Eye Institute In connection with the Company’s research and collaboration agreement, as amended, with LEI (the “LEI Agreement”), the Company agreed to issue a warrant to purchase a certain number of the Company’s common stock upon the achievement of each milestones as set forth in the LEI Agreement. During the year ended December 31, 2015, the Company issued a warrant to purchase 40,000 shares of its common stock with an exercise price of $10.51 per share to LEI. This common stock warrant is exercisable immediately, and expires on October 15, 2020. The estimated the fair value of this warrant was approximately $0.2 million and was recorded within research and development expenses in the Company’s consolidated statement of operations and comprehensive loss and additional paid-in capital in the Company’s consolidated balance sheet for the year ended December 31, 2015. The fair value of the warrant was calculated using the Black-Scholes valuation model, and was based on the closing price of common stock on the issuance date of $8.35 per share, contractual term of the warrant of 5 years, a risk-free interest rate of 1.34%, an expected volatility of 75% and a 0% expected dividend yield. Additionally, in September 2017, the Company issued a warrant to purchase 40,000 shares of its common stock with an exercise price of $3.65 per share to LEI. This common stock warrant is exercisable immediately, and expires on September 29, 2022. The estimated the fair value of this warrant was approximately $0.1 million and the fair value of this warrant was recorded as research and development expenses in the Company’s consolidated statement of operations and comprehensive loss and additional paid-in capital in the Company’s consolidated balance sheet for the year ended December 31, 2017. The fair value of the warrant was calculated using the Black-Scholes valuation model, and was based on the closing price of common stock on the issuance date of $3.65 per share, contractual term of the warrant of 5 years, a risk-free interest rate of 1.89%, an expected volatility of 91% and a 0% expected dividend yield. TAP Warrant. In July 2016, in connection with the TAP financing agreement (see Note 10), the Company issued a warrant to purchase 10,000 shares of its common stock exercisable anytime during five years from the issuance date at an exercise price of $4.33 per share. The estimated fair value of this warrant was $26,000 at the issuance date using the Black-Scholes valuation model with the following assumptions: exercise price of $4.33 per share, expected term of the warrant of 5 years, a risk-free interest rate of 1.07%, an expected volatility of 72% and a 0% expected dividend. The fair value of TAP warrant was recorded to non-current liabilities and additional paid-in-capital in the Company’s consolidated balance sheet for the year ended December 31, 2016.

Stock Plans

Stock Plans	12 Months Ended
	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock Plans	13. Stock Plans On December 26, 2006, the Company adopted the 2006 Equity Incentive Plan, which was amended by the board of directors on November 15, 2012 (the “2006 Plan”). The 2006 Plan allowed for the granting of incentive stock options (“ISOs”) and non-qualified stock options (“NSOs”) to the employees, members of the board of directors and consultants of the Company. ISOs were granted only to the Company’s employees, including officers and directors who are also employees. NSOs were granted to the employees and consultants. In July 2014, the Company’s board of directors and its stockholders approved the establishment of the 2014 Equity Incentive Award Plan (the “2014 Plan”). Options may no longer be issued under the 2006 Plan after July 30, 2014. In addition, the 2014 Plan provides for annual increases in the number of shares available for issuance thereunder on the first business day of each fiscal year, beginning with the year ended December 31, 2015, equal to four percent (4%) of the number of shares of the Company’s common stock outstanding as of such date or a lesser number of shares as determined by the Company’s board of directors. In October 2017, the Company adopted the 2017 Inducement Plan (the “Inducement Plan”). The Company reserved 600,000 shares for issuance pursuant to stock options and RSUs under the Inducement Plan. The only persons eligible to receive grants of stock options and RSUs under Inducement Plan are individuals who satisfy the standards for inducement grants under Nasdaq guidance that is, generally, a person not previously an employee or director of Adverum, or following a bona fide period of non-employment, as an inducement material to the individual's entering into employment with Adverum. The 2006 Plan, 2014 Plan and Inducement Plan are referred to collectively as the Plans. As of December 31, 2017, a total of 17,167,856 shares of common stock were reserved for issuance and 2,830,010 shares were available for future grants under the Plans. Stock Options Stock options under the 2014 Plan and the Inducement Plan may be granted for periods of up to 10 years and at prices no less than 100% of the estimated fair value of the shares on the date of grant as determined by the board of directors, provided, however, that the exercise price of an ISO and NSO granted to a 10% stockholder may not be less than 110% of the estimated fair value of the shares on the date of grant. Stock options granted to employees and non-employees generally vest ratably over four years. The following table summarizes stock option activity under the Company’s stock plans and related information: (In thousands, except exercise prices and years) Options Outstanding Weighted- Average Exercise Price Weighted- Average Remaining Contract Life (in years) Aggregate Intrinsic Value (a) Balance at December 31, 2016 7,449 $ 4.46 8.4 $ 11,837 Granted 1,989 2.85 Exercised (1,808 ) 0.20 Cancelled/forfeited (935 ) 8.87 Balance at December 31, 2017 6,695 $ 4.51 7.4 $ 9,539 Vested and expected to vest as of December 31, 2017 6,695 $ 4.51 7.4 $ 9,539 Exercisable at December 31, 2017 3,202 $ 4.96 6.1 $ 6,207 (a) The aggregate intrinsic value is calculated as the difference between the stock option exercise price and the closing price of the Company’s common stock of $3.5 per share as of December 31, 2017. In June 2017 and 2016, the Company granted 150,000 stock options and 518,000 stock options, respectively, outside the Plans to its certain executive officers. In December 2015, the Company granted 910,000 stock options outside of the Plans to its new Principal Executive Officer. The total intrinsic value of stock options exercised during the years ended December 31, 2017, 2016 and 2015 were $4.8 million, $7.1 million and $11.2 million, respectively. Stock Options Granted to Employees. The fair value of each stock option issued to employees was estimated at the date of grant using the Black-Scholes valuation model with the following weighted-average assumptions: Options Employee Stock Purchase Plan Years ended December 31, Years ended December 31, 2017 2016 2015 2017 2016 2015 Expected volatility 82 % 81 % 79 % 52 % 70 % 74 % Expected term (in years) 6.0 6.1 6.1 0.5 0.5 0.5 Expected dividend yield — — — — — — Risk-free interest rate 1.9 % 1.8 % 1.7 % 1.3 % 0.6 % 0.1 % The weighted-average fair values of options granted during the years ended December 31, 2017, 2016 and 2015 were $1.99, $3.35 and $15.91, respectively. As of December 31, 2017, there was $8.9 million of unrecognized stock-based compensation expense related to employee stock options that is expected to be recognized over a weighted-average period of 2.2 years. Stock Options Granted to Non-Employees. Stock-based compensation expense related to stock options granted to non-employees is recognized as the stock options are earned. The Company believes that the estimated fair value of the stock options is more readily measurable than the fair value of the services rendered. The following table presents the weighted-average assumptions used in the Black-Scholes valuation model to estimate the fair value of non-employee stock options: Years ended December 31, 2017 2016 2015 Expected volatility 84 % 82 % 81 % Expected term (in years) 8.8 9.1 3.3 Expected dividend yield — — — Risk-free interest rate 2.3 % 2.1 % 1.0 % As of December 31, 2017, unrecognized stock-based compensation expense related to unvested non-employees stock options was approximately $0.4 million, which is expected to be recognized over a weighted-average period of 1.5 years, based on the estimated fair value at December 31, 2017. RSUs RSUs are share awards that entitle the holder to receive freely tradable shares of the Company’s common stock upon vesting. The fair value of RSUs is based upon the closing sales price of the Company’s common stock on the grant date. RSUs granted to employees generally vest over a two to four-year period. The following table summarizes the RSU activity under the Company’s stock plans and related information: (In thousands, except grant date fair value and years) Number of Units (in thousands) Weighted- Average Grant Date Fair Value (in dollars) Weighted- Average Remaining Contractual Term (in years) Balance at December 31, 2016 1,049 $ 5.47 1.7 Granted 2,543 2.81 Vested and released (307 ) 6.43 Forfeited (770 ) 3.58 Balance at December 31, 2017 2,515 $ 3.24 1.6 The weighted-average grant-date fair value of RSUs granted during the years ended December 31, 2017, 2016 and 2015 were $2.81, $4.40 and $13.85, respectively. During the years ended December 31, 2017, 2016 and 2015, total fair value of RSUs vested was $2.0 million, $1.7 million and $1.4 million, respectively. The number of RSUs vested includes shares of common stock that the Company withheld on behalf of employees to satisfy the minimum statutory tax withholding requirements. As of December 31, 2017, there was $6.4 million of unrecognized compensation cost related to unvested RSUs that is expected to be recognized over a weighted-average period of 2.9 years. ESPP In July 2014, the Company approved the establishment of the 2014 Employee Stock Purchase Plan (the “ESPP”). The Company reserved 208,833 shares of its common stock for issuance and provided for annual increases in the number of shares available for issuance on the first business day of each fiscal year, beginning in 2015, equal to the lesser of one percent (1%) of the number of shares of the Company’s common stock outstanding as of such date or a number of shares as determined by the Company’s board of directors. During the years ended December 31, 2017, 74,642 shares were issued under the ESPP. As of December 31, 2017, a total of 962,095 shares of common stock were available for future issuance under the ESPP. As of December 31, 2017, unrecognized compensation cost related to the ESPP was immaterial. Stock-Based Compensation Recognized in the Consolidated Statement of Operations and Comprehensive Loss The following table presents, by operating expense, the Company’s stock-based compensation expense: Years ended December 31, 2017 2016 2015 (In thousands) Research and development $ 5,253 $ 6,616 $ 4,009 General and administrative 3,470 4,800 6,447 Restructuring charges — — 1,049 Total share-based compensation expense $ 8,723 $ 11,416 $ 11,505 During the year ended December 31, 2016, stock-based compensation expense included one-time charges of $1.5 million, which was recorded in general and administrative expense, and $1.4 million, which was recorded in research and development expense, related to stock award modifications in connection with the separation agreements for four of Company’s executive officers. During the year ended December 31, 2015, the Company recorded a one-time stock-based compensation expense of $2.4 million related to the cancellation of unvested stock options in connection with the separation agreement for one of the Company’s executive officers.

401(k) Savings Plan

401(k) Savings Plan	12 Months Ended
	Dec. 31, 2017
Postemployment Benefits [Abstract]
401(k) Savings Plan	14. 401(k) Savings Plan The Company established a defined-contribution savings plan under Section 401(k) of the Code. The 401(k) Plan covers all employees who meet defined minimum age and service requirements, and allows participants to defer a portion of their annual compensation on a pretax basis. The amount of contributions that the Company made to the 401(k) Plan during the year ended December 31, 2017, 2016 and 2015 was $0.3 million, $0.3 million and $0.1 million, respectively

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	15. Income Taxes The Company recorded $0.8 million income tax benefit related to the change in the deferred tax liabilities balance due to intangible assets impairment recognized in the fourth quarter of 2016 and The following table presents domestic and foreign components of loss before provision for income taxes: Years ended December 31, 2017 2016 2015 (In thousands) U.S. $ (36,923 ) $ (96,498 ) $ (47,235 ) Foreign (19,224 ) (18,024 ) (218 ) Loss before income taxes $ (56,147 ) $ (114,522 ) $ (47,453 ) A reconciliation of income tax expense computed at the statutory federal income tax rate of 34% to income taxes as reflected in the financial statements is as follows: Years ended December 31, 2017 2016 2015 (In thousands) Federal income tax expense at statutory rate $ (19,090 ) $ (38,938 ) $ (16,134 ) Non-deductible foreign research expenses 19 21 45 Stock-based compensation (76 ) 1,356 1,418 Non-deductible expenses 61 985 295 Goodwill impairment — 16,834 — Research and development tax credits (507 ) (326 ) (910 ) Change in valuation allowance 7,935 16,062 15,277 Foreign rate differential 1,495 3,231 9 Rate change 10,163 — — Total tax benefit $ — $ (775 ) $ — Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. The following table presents significant components of the Company’s deferred tax assets: As of December 31, 2017 2016 2015 (In thousands) Deferred tax assets: Net operating loss carryforwards $ 27,142 $ 15,785 $ 19,889 Accruals, reserve and other 2,586 3,071 2,681 Stock-based compensation 3,623 3,848 4,328 Tax credit carryforwards 1,774 574 1,945 Property and equipment 274 54 — Intangibles 64 68 45 Total deferred tax assets before valuation allowance 35,463 23,400 28,888 Valuation allowance (35,463 ) (23,400 ) (28,839 ) Total deferred tax assets — — 49 Deferred tax liabilities: Property and equipment — — (49 ) IPR&D (1,250 ) (1,250 ) — Total deferred tax liabilities $ (1,250 ) $ — $ (49 ) Net deferred tax assets $ — $ — $ — On December 22, 2017, the U.S. government enacted comprehensive tax legislation, commonly known as the Tax Cuts and Jobs Act of 2017 (the “Act”), which significantly reforms the Internal Revenue Code of 1986, as amended. The Act contains broad and complex changes to corporate taxation, including in part reduction of the U.S. federal corporate tax rate from 35% to 21%, requires companies to pay a one-time transition tax on earnings of certain foreign subsidiaries that were previously considered permanently reinvested, and creates new taxes on certain foreign sourced earnings. As of December 31, 2017, the Company was able to determine a reasonable estimate, namely the one-time transition tax and the remeasurement of deferred tax at the new tax rate. The Company did not recognize any provisional tax expense due to its significant operating losses. The effect on the Company’s deferred tax balance due to the change of net tax rate was $10.2 million and was fully offset by its valuation allowance, therefore, there was no net effect to the Company’s effective tax rate for the year ended December 31, 2017. The one-time transition tax is based on the Company’s post-1986 foreign earnings and profits which the Company had previously excluded from U.S. income taxes due to its position that it would permanently reinvest its future earnings. The one-time transition tax is applied at a 15.5% tax rate on cash assets and an 8% tax rate for other specified assets. Since the Company’s foreign operations incurred aggregated losses, the Company did not record provisional amount for its one-time transition tax liability for its foreign subsidiaries due to overall cumulative foreign losses. Additionally, the SEC staff has issued SAB 118, which allows the Company to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. On December 22, 2017, Staff Accounting Bulletin No. 118 ("SAB 118") was issued to address the application of U.S. GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the Act. Because the Company is still in the process of analyzing certain provisions of the Act including the application of new executive compensation limitation provisions under Internal Revenue Section 162(m) in accordance with SAB 118, the Company determined that the adjustment to its deferred taxes was a provisional amount and a reasonable estimate at December 31, 2017. The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets. Based on the Company’s history of operating losses, the Company has concluded that it is more likely than not that the benefit of its deferred tax assets will not be realized. Accordingly, the Company has provided a full valuation allowance for deferred tax assets as of December 31, 2017, 2016 and 2015. The valuation allowance increased approximately $12.0 million during the years ended December 31, 2017 and decreased approximately $5.3 million during the year ended December 31, 2016. As of December 31, 2017, the Company had U.S. federal net operating losses (“NOLs”) carryforwards of approximately $53.2 million to offset future federal income. NOLs expire at various years beginning with 2036. As of December 31, 2017, the Company also had U.S. state NOL carryforwards of approximately $37.8 million to offset future state income. U.S. State NOLs expire at various years beginning with 2036. At December 31, 2017, the Company also had approximately $44.1 million of foreign net operating loss carryforwards which may be available to offset future foreign income; these carryforwards do not expire. As of December 31, 2017, the Company had federal research and development tax credit carryforwards of approximately $0.8 million available to reduce future tax liabilities which expire at various years beginning with 2036. As of December 31, 2017, the Company had state credit carryforwards of approximately $1.0 million available to reduce future tax liabilities which do not expire. Under Section 382 of the Internal Revenue Code of 1986, as amended (Code), the Company’s ability to utilize NOL carryforwards or other tax attributes, such as research tax credits, in any taxable year may be limited if the Company experiences an “ownership change.” Generally, a Section 382 ownership change occurs if there is a cumulative increase of more than 50 percentage points in the stock ownership of one or more stockholders or groups of stockholders who owns at least 5% of a corporation’s stock within a specified testing period. Similar rules may apply under state tax laws. The Company believes that it has experienced ownership changes under Section 382, which will result in limitations in the Company’s ability to utilize net operating losses and credits. As a result, the amount of the NOLs and research and credit carryforwards presented in the Company’s consolidated financial statements are limited and will expire unutilized, and the Company removed a significant amount of NOLs and credits from its deferred taxes. The Company files income tax returns in the U.S., and state and foreign jurisdictions. The federal, state and foreign income tax returns are open under the statute of limitations subject to tax examinations for the tax years ended December 31, 2012 through December 31, 2017. To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the Internal Revenue Service, state or foreign tax authorities to the extent utilized in a future period. The Company has total unrecognized tax benefits as of December 31, 2017, 2016 and 2015 of approximately $2.7 million, $2.1 million and $1.6 million, respectively. No amount of the unrecognized tax benefits, if recognized, would reduce the Company’s annual effective tax rate because the benefits are in the form of deferred tax assets for which a full valuation allowance has been recorded. The Company does not anticipate a significant change to its unrecognized tax benefits over the next twelve months. A reconciliation of the unrecognized tax benefits is as follows: Years ended December 31, 2017 2016 2015 (In thousands) Unrecognized tax benefits as of the beginning of the year $ 2,157 $ 1,568 $ 471 Increase (decrease) related to prior year tax provisions — — 172 Increase related to current year tax provisions 588 589 925 Unrecognized tax benefits as of the end of the year $ 2,745 $ 2,157 $ 1,568 The Company recognizes interest and penalties related to uncertain tax positions in income tax expense. As of December 31, 2017 and 2016, the Company had no accrued interest or penalties related to uncertain tax positions and no amounts have been recognized in the Company’s consolidated statements of operations and comprehensive loss. There are no ongoing examinations by taxing authorities at this time.

Net Loss per Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Net Loss per Share	16. Net Loss per Share The following common stock equivalents outstanding at the end of the periods presented were excluded from the calculation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect: As of December 31, 2017 2016 2015 (In thousands) Stock options 6,695 7,449 5,494 Restricted stock units 2,515 1,049 632 ESPP 71 47 42 Warrants to purchase common stock 90 50 40 9,371 8,595 6,208

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	17. Subsequent Events Editas Agreement On January 25, 2018, the Company and Editas extended the research collaboration, option and license agreement. In consideration for extending the agreement, Editas made a one-time payment, non-refundable cash payment of $0.5 million to the Company in February 2018. Under the terms of the agreement, as amended, Editas may exercise the option with respect to a designated initial indication until September 30, 2018. With respect to the four other indications, Editas may exercise the option until the fourth anniversary of the effective date, provided that the option will expire on the third anniversary of the effective date if Editas has not exercised the option with respect to the initial indication or any other indication by such date. Upon Editas’ timely exercise of the option with respect to the designated initial indication, Editas will pay the Company a $1.3 million fee. For the first additional indication for which Editas timely exercises its option, Editas will pay the Company a $1.5 million fee. Upon each subsequent exercise of the option, Editas will pay the Company a $1.0 million fee per indication. Follow-on Offerings of Common Stock In January 2018, the Company issued and sold a total of 1,419,893 shares of its common stock at market prices under the 2017 stock offering agreement and raised total net proceeds of approximately $5.7 million, net of issuance costs. In February 2018, the Company completed an underwritten public offering for the sale of 10,222,235 shares of its common stock and raised total net proceeds of approximately $64.3 million, after discounts and other issuance costs.

Selected Quarterly Financial In

Selected Quarterly Financial Information	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Information	18. Selected Quarterly Financial Information (Unaudited) The Company’s quarterly consolidated results of operations are shown below: Quarterly Results of Operations Three Months Ended March 31, 2017 June 30, 2017 September 30, 2017 December 31, 2017 (In thousands, except per share amounts) Revenue $ 462 $ 463 $ 463 $ 461 Total operating expenses (1) (17,050 ) (12,556 ) (15,034 ) (16,056 ) Net loss (16,099 ) (11,430 ) (13,829 ) (14,789 ) Basic and diluted net loss per share (0.38 ) (0.27 ) (0.32 ) (0.32 ) Quarterly Results of Operations Three Months Ended March 31, 2016 June 30, 2016 September 30, 2016 December 31, 2016 (In thousands, except per share amounts) Revenue $ 265 $ 307 $ 395 $ 488 Total operating expenses (2)(3)(4) (15,773 ) (62,189 ) (14,902 ) (23,875 ) Net loss (15,392 ) (61,660 ) (14,301 ) (22,394 ) Basic and diluted net loss per share (0.57 ) (1.76 ) (0.35 ) (0.54 ) (1) During the year ended December 31, 2017, the Company recorded a total of $2.0 million of estimated costs associated with the termination of MSA with Cornell University (see Note 7). During the three months ended March 31, 2017, the Company recorded a total of $2.3 million of these estimated costs, which was subsequently adjusted by $0.3 million during the three months ended June 30, 2017. (2) During the year ended December 31, 2016, the Company performed a two-step goodwill impairment analysis and recorded $49.1 million and $0.4 million of goodwill impairment charge, respectively, during the three months ended June and September, 2016. ( 3 ) During the three months ended December 31, 2016, the Company performed its annual assessment of its intangible assets, ADVM-043 and ADVM-053, and recorded a total of $11.2 million of impairment charge related to its intangible assets. ( 4 ) During the three months ended March 31, 2016, two officers of the company resigned and the Company recorded $2.2 million of one-time stock-based compensation charge related to the accelerated vesting of their stock-based awards. Basic and diluted net loss per share is computed independently for each of the quarters presented. Therefore, the sum of quarterly basic and diluted per share amounts may not equal annual basic and diluted net loss per share amounts.

Summary of significant accoun26

Summary of significant accounting policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	Basis of Presentation and Principles of Consolidation — The Company’s consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”). The consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All inter-company transactions and balances have been eliminated in consolidation.
Use of Estimates	Use of Estimates —The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, and the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of income and expenses during the reporting period. The Company bases estimates and assumptions on historical experience when available and on various factors that it believes to be reasonable under the circumstances. On an ongoing basis, the Company evaluates its estimates and assumptions, including those related to research and development expense accruals, stock-based compensation expense, income taxes, intangible asset, fair values of financial instruments and fair value of common stock warrants. The Company’s actual results may differ from these estimates under different assumptions or conditions. There have been no significant changes from the Company’s original estimates in any periods presented.
Foreign Currency Translation	Foreign Currency Translation —The Company’s consolidated financial statements are prepared in U.S. dollars. The Company’s foreign subsidiaries use the Euro and Australian dollar as their functional currencies and maintain their records in their local currencies, except its Ireland subsidiary that uses the U.S. dollar as its functional currency. Assets and liabilities are re-measured at exchange rates in effect at the end of the reporting period for the Company’s French and Australian subsidiaries, and at historical exchange rates for its Irish subsidiary. Equity is measured at historical rates and income and expenses are re-measured at average exchange rates for the reporting period. The resulting foreign currency translation adjustment is recorded in accumulated other comprehensive loss in the consolidated balance sheet. Transactions denominated in foreign currency are translated at exchange rates at the date of transaction with foreign currency gains (losses) recorded in other income (expense), net in the consolidated statements of operations and comprehensive loss.
Cash and Cash Equivalents	Cash and Cash Equivalents —The Company considers all highly liquid investments purchased with original maturities of three months or less at the date of purchase to be cash equivalents. Cash and cash equivalents include cash held in banks, money market accounts and highly liquid debt securities. Cash equivalents are stated at fair value.
Short-Term Investments	Short-Term Investments —All short-term investments, which consist of debt securities and certificates of deposit, have been classified as “available for sale” and are carried at fair value. Unrealized gains and losses, net of any related tax effects, are excluded from earnings and are included in other comprehensive loss and reported as a separate component of stockholders’ equity until realized. Realized gains and losses and declines in value judged to be other than temporary, if any, on available-for-sale securities are included in other income (expense), net in the Company’s consolidated statements of operations and comprehensive loss. The cost of securities sold is based on the specific-identification method. The amortized cost of securities is adjusted for amortization of premiums and accretion of discounts to maturity. Interest on short-term investments is included in other income, net in the Company’s consolidated statements of operations and comprehensive loss. In accordance with the Company’s investment policy, management invests to diversify credit risk and only invests in securities with high credit quality, including U.S. government securities. The Company periodically evaluates whether declines in the fair value of its investments below their cost are other than temporary. The evaluation includes consideration of the cause of the impairment, including the creditworthiness of the security issuers, the number of securities in an unrealized loss position, the severity and duration of the unrealized losses, whether the Company has the intent to sell the securities, and whether it is more likely than not that the Company will be required to sell the securities before the recovery of their amortized cost basis. If the Company determines that the decline in fair value of an investment is below its accounting basis and this decline is other than temporary, the Company would reduce the carrying value of the security it holds and records a loss for the amount of such decline. The Company has not recorded any realized losses or declines in value judged to be other than temporary on its investments.
Segment Reporting	Segment Reporting —The Company operates and manages its business as one reporting and operating segment, which is the business of developing and commercializing gene therapeutics. The Company’s chief executive officer, who is the chief operating decision maker, reviews financial information on an aggregate basis for purposes of allocating resources and evaluating financial performance.
Concentrations of Credit Risk and Other Uncertainties	Concentrations of Credit Risk and Other Uncertainties —Financial instruments that potentially subject the Company to significant concentrations of credit risk consists primarily of cash, cash equivalents and short-term investments. Risks associated with cash, cash equivalents, short-term investments are mitigated by the Company’s investment policy, which limits the Company’s investing to only those marketable securities rated at least A-1/P-1 Short Term Rating and A/A2 Long Term Rating, as determined by independent credit rating agencies. Management believes that the Company is not exposed to significant credit risk. The Company is subject to certain risks and uncertainties, including, but not limited to changes in any of the following areas that the Company believes could have a material adverse effect on future financial position or results of operations: ability to obtain future financing; regulatory approval and market acceptance of, and reimbursement for, the Company’s product candidates; performance of third-party clinical research organizations and manufacturers; development of sales channels; protection of the intellectual property; litigation or claims against the Company based on intellectual property, patent, product, regulatory or other factors; and the Company’s ability to attract and retain employees necessary to support the growth.
Property and Equipment	Property and Equipment —Property and equipment are recorded at cost, net of accumulated depreciation and amortization. Depreciation is recorded using the straight-line method over the estimated useful lives of the assets, generally three to five years. Leasehold improvements are capitalized and amortized over the shorter period of their expected lives or the lease term. Major replacements and improvements are capitalized, while general repairs and maintenance are expensed as incurred.
Valuation of Long-Lived Assets and Purchased Intangible Assets	Valuation of Long-Lived Assets and Purchased Intangible Assets —The Company evaluates the carrying value of amortizable long‑lived assets, whenever events, or changes in business circumstances or the planned use of long‑lived assets indicate that their carrying amounts may not be fully recoverable or that their useful lives are no longer appropriate. If these facts and circumstances exist, the Company assesses for recovery by comparing the carrying values of long‑lived assets with their future undiscounted net cash flows. If the comparison indicates that impairment exists, long‑lived assets are written down to their respective fair values based on discounted cash flows. Significant management judgment is required in the forecasting of future operating results that is used in the preparation of expected undiscounted cash flows. If management’s assumptions about future operating results were to change as a result of events or circumstances, the Company may be required to record an impairment loss on these assets. No impairment indicators were noted for the Company’s amortizable long-lived assets, fixed assets, in the periods presented. The Company also evaluates the carrying value of intangible asset (not subject to amortization) related to in‑process research and development (“IPR&D”) asset, which is considered to be indefinite‑lived until the completion or abandonment of the associated research and development efforts. Accordingly, amortization of the IPR&D assets will not occur until the product reaches commercialization. During the period the intangible asset is considered indefinite‑lived, it will be tested for impairment on an annual basis, as well as between annual tests if the Company becomes aware of any events occurring or changes in circumstances that would indicate that the fair value of the IPR&D asset is less than its carrying amount. If and when development is complete, which generally occurs when regulatory approval to market the product is obtained, the associated IPR&D asset would be deemed definite‑lived and would then be amortized based on its estimated useful life at that point in time based on respective patent term. If a potential impairment exists, an impairment loss is measured as the excess of the asset’s carrying value over its fair value. During the year ended December 31, 2016, the Company recorded an impairment charge of $11.2 million related to its intangible assets (see Note 3).
Financial Liabilities	Financial Liabilities — During the year ended December 31, 2016, the Company entered into a sponsored research agreement with The Alpha-1 Project, Inc. (the “TAP”) with an embedded derivative, the Company determined to account for this financial liability at fair value and recorded as other non-current liabilities in its consolidated balance sheets (see Note 6). Change in fair value is recorded within other income, net in the Company’s consolidated statement of operations and comprehensive loss.
Revenue Recognition	Revenue Recognition —The Company has primarily generated revenue through license, research and collaboration arrangements with its strategic partners. The terms of these types of agreements may include (i) licenses to Adverum technology, (ii) research In arrangements involving the delivery of more than one element, each required deliverable is The arrangement’s consideration that is fixed or determinable is allocated to each separate unit Payments or reimbursements for the Company’s research and development efforts for the arrangements where The Company’s collaboration and license agreements may include contingent payments related to Collaboration and License Revenue Editas Agreement —In August 2016, the Company entered into a collaboration, option and license agreement with Editas Medicine, Inc. (“Editas”) (see Note 7). Under the terms of the agreement, the Company received initial payments of $1.0 million that included $0.5 million for research services during the year ended December 31, 2016. As the agreement provides for multiple deliverables, the Company accounts for this agreement as a multiple elements revenue arrangement. At the inception of the agreement, identified deliverables include research services, manufacturing of viral vectors for research, participation in joint research committee and exclusivity during the option period. These deliverables did not appear to have a standalone value and were combined into one unit of accounting. Options for each indication to license the Company’s AAV vector are considered substantive options and do not include significant incremental discounts. Therefore, they are not considered as deliverables under the agreement. In January 2018, the Company and Editas extended the research collaboration, option and license agreement (see Note 7). In consideration for extending the agreement, Editas made a one-time payment, non-refundable cash payment of $0.5 million to the Company in February 2018. The Company allocated the $1.0 million received to a single unit of accounting identified in the arrangement. The Company expects to recognize $1.0 million ratably over the associated period of performance, which is the maximum research period of three years. As there is no discernible pattern of performance and/or objectively measurable performance measures do not exist, the Company recognizes revenue on a straight-line basis. During the years ended December 31, 2017 and 2016, the Company recognized $0.3 million and $0.1 million, respectively, as collaboration and license revenue related to Editas agreement. Regeneron Agreement —In May 2014, the Company entered into a research collaboration and license agreement with Regeneron Pharmaceuticals, Inc. (“Regeneron”) to discover, develop and commercialize novel gene therapy products for the treatment of ophthalmologic diseases (see Note 7). Under the terms of the agreement, during the year ended December 31, 2014, the Company received initial upfront non-refundable cash payments of $8.0 million that included payment for research license fees, prepaid collaboration research costs and the time-limited right of first negotiation for license to develop and commercialize AVA-101. As the agreement provides for multiple deliverables, the Company accounts for this agreement as a multiple elements revenue arrangement. If deliverables did not appear to have a standalone value, they were combined with other deliverables into a unit of accounting with a standalone value. The Company allocated the $8.0 million to the relative fair value of the two units of accounting identified in the arrangement. The Company recognizes $6.5 million allocated to the first unit of accounting for the research licenses and related research and development services ratably over the associated period of performance, which is the maximum research period of eight years. As there was no discernible pattern of performance and/or objectively measurable performance measures did not exist for the first unit of accounting, the Company recognizes revenue on a straight-line basis over the eight years performance period. The remaining $1.5 million allocated to the second unit of accounting for the time-limited right of first negotiation for license to develop and commercialize AVA-101 was deferred. On November 2, 2015, Regeneron notified the Company that it did not exercise this right of first negotiation and the Company recognized the entire $1.5 million as revenue during the year ended December 31, 2015. As original research budget was fully used in the fourth quarter of 2015, the Company and Regeneron will agree on the reimbursement of additional research expenses annually. The Company invoices for services performed quarterly. These additional research fees are added to the research licenses and related research and development services unit of accounting, recorded as deferred revenue and recognized to revenue over the remaining research term. During the years ended December 31, 2017, 2016 and 2015, the Company recognized $1.5 million, $1.4 million and $0.8 million, respectively, related to the research licenses and research and development services unit of accounting in the Regeneron agreement.
Research and Development Expenses	Research and Development Expenses —Research and development expenses are charged to expense as incurred. Research and development expenses include primarily personnel-related costs, stock-based compensation expense, laboratory supplies, consulting costs, external contract research and development expenses, including expenses incurred under agreements with CROs, the cost of acquiring, developing and manufacturing clinical study materials, and overhead expenses, such as rent, equipment depreciation, insurance and utilities. Advance payments for goods or services for future research and development activities are deferred and expensed as the goods are delivered or the related services are performed. The Company estimates preclinical studies and clinical trial expenses based on the services performed pursuant to contracts with research institutions and clinical research organizations that conduct and manage preclinical studies and clinical trials on the Company’s behalf. In accruing service fees, the Company estimates the time period over which services will be performed and the level of effort to be expended in each period. These estimates are based on communications with the third-party service providers and the Company’s estimates of accrued expenses and on information available at each balance sheet date. If the actual timing of the performance of services or the level of effort varies from the estimate, the Company will adjust the accrual accordingly. There have been no significant changes from the Company’s original estimates in any of the periods presented. The Company received tax credits from the Australian and French governments in connection with certain research costs incurred in conducting research by the Company’s Australian and French subsidiaries. These refunds do not depend on the taxable income or tax position of the Company and therefore the Company does not account for them under an income tax accounting model. The Company recognizes such tax credits in the period when qualified expenses are incurred as a reduction of research expenses. The Company has recorded the reimbursement of $0.1 million, $0.3 million and $0.1 million from the tax authorities as a reduction of research and development expense in the consolidated statements of operations and comprehensive loss in the years ended December 31, 2017, 2016 and 2015, respectively.
Fair Value Measurements	Fair Value Measurements —Fair value is defined as 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. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. The carrying amounts of the Company’s financial instruments, including cash equivalents, prepaid and other current assets, accounts payable, accrued expenses and other current liabilities approximate their fair values due to their short-term maturities. Refer to Note 6 for the methodologies and assumptions used in valuing financial instruments.
Stock-Based Compensation Expense	Stock-Based Compensation Expense —Stock-based compensation expense related to stock awards to employees is measured at fair value of the award on the date of the grant. The Company estimates the grant-date fair value, and the resulting stock-based compensation expense, using the Black-Scholes valuation model for stock options and using intrinsic value, which is the closing price of its common stock on the date of the grant, for the restricted stock units (“RSUs”). The fair value of the award that is ultimately expected to vest is recognized as expense on a straight-line basis over the requisite service period, which is generally the vesting period. As of January 1, 2017, the Company adopted Accounting Standard Update (“ASU”) No. 2016-09 and elected to account for forfeitures as they occur using a modified retrospective transition method, which requires the Company to record cumulative-effect adjustment to accumulated deficit. The Company determined that the impact of this adoption was immaterial and no adjustments were recorded in its consolidated financial statements. Prior to the adoption of ASU No. 2016-09, stock-based compensation expense recognized for the portion of the award that is expected to vest was reduced by an estimated forfeiture rate. The Black-Scholes valuation model requires the use of following assumptions: Expected Term —The expected term assumption represents the period that the Company’s stock-based awards are expected to be outstanding and is determined using the simplified method. Expected Volatility —Expected volatility is estimated using comparable public companies’ volatility for similar terms. Expected Dividend —The Black-Scholes valuation model calls for a single expected dividend yield as an input. The Company has never paid dividends and has no plans to pay dividends. Risk-Free Interest Rate —The risk-free interest rate is based on the U.S. Treasury zero-coupon issues in effect at the time of grant for periods corresponding with the expected term of option. Stock-based compensation expense related to awards granted to non-employees is recognized based on the then-current fair value at each measurement date over the associated service period of the award, which is generally the vesting term, using the accelerated attribution method. The fair value of non-employee stock options is estimated using the Black-Scholes valuation model with assumptions generally consistent with those used for employee stock options, with the exception of the expected term, which is the remaining contractual life at each measurement date. Refer to Note 13 for more information on assumptions used in estimating stock-based compensation expense.
Income Taxes	Income Taxes —The Company accounts for income taxes using the asset and liability method. The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. Deferred tax assets and liabilities are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. In evaluating the ability to recover its deferred income tax assets, the Company considers all available positive and negative evidence, including its operating results, ongoing tax planning and forecasts of future taxable income on a jurisdiction-by-jurisdiction basis. In the event the Company determines that it would be able to realize its deferred income tax assets in the future in excess of their net recorded amount, it would make an adjustment to the valuation allowance that would reduce the provision for income taxes. Conversely, in the event that all or part of the net deferred tax assets are determined not to be realizable in the future, an adjustment to the valuation allowance would be charged to earnings in the period when such determination is made. As of December 31, 2017 and 2016, the Company has recorded a full valuation allowance on its deferred tax assets. Tax benefits related to uncertain tax positions are recognized when it is more likely than not that a tax position will be sustained during an audit. Interest and penalties related to unrecognized tax benefits are included within the provision for income tax.
Comprehensive Loss	Comprehensive Loss —Comprehensive loss is comprised of net loss and other comprehensive income (loss). Other comprehensive income (loss) consists of foreign currency translation adjustments related to translation of the financial statements of the Company’s Australia and France subsidiaries and unrealized gain (loss) on marketable securities. The Company did not have reclassifications from other comprehensive income (loss) to the income (loss) during the years ended December 31, 2017, 2016 and 2015.
Basic and Diluted Net Loss Per Share	Basic and Diluted Net Loss Per Share — Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding for the period. Diluted net loss per share is computed by giving effect to all potential dilutive common stock equivalents outstanding for the period using the treasury stock method. Outstanding stock options, RSUs, ESPP and warrants are considered to be common stock equivalents and are only included in the calculation of diluted net loss per share when their effect is dilutive.
Recent Accounting Standard Update Not Yet Effective	Recent Accounting Standard Update Not Yet Effective — In May 2014, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606), which supersedes the revenue recognition requirements in ASC 605 , Revenue Recognition . This ASU is based on the principle that revenue is recognized to depict the transfer of goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. ASU 2014-09 defines a five-step process to achieve this core principle and, in doing so, it is possible more judgment and estimates may be required within the revenue recognition process than required under existing U.S. GAAP including identifying performance obligations in the contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. ASU 2014-09 is required to be adopted, using either of two methods: (i) retrospective to each prior reporting period presented with the option to elect certain practical expedients as defined within ASU 2014-09; or (ii) retrospective with the cumulative effect of initially applying ASU 2014-09 recognized at the date of initial application and providing certain additional disclosures. In July 2015, the FASB voted to approve a one-year deferral of the effective date to December 15, 2017 for interim and annual reporting periods beginning after that date and permitted early adoption of the standard, but not before the original effective date of December 15, 2016 . The FASB issued supplemental adoption guidance and clarification to ASU 2014-09 in March 2016, April 2016, May 2016, December 2016 and November 2017 within ASU 2016-08 Revenue From Contracts With Customers: Principal vs. Agent Considerations , ASU 2016-10 Revenue From Contracts with Customers: Identifying Performance Obligations and Licensing , ASU 2016-12 Revenue from Contracts with Customers: Narrow-Scope Improvements and Practical Expedients , ASU 2016-20 Technical Corrections and Improvement to Topic 606 – Revenue from Contracts with Customers and ASU 2017-14 Reporting Comprehensive Income (Topic 220), Revenue Recognition (Topic 605), and Revenue from Contracts with Customers (Topic 606), respectively. The Company will adopt the new standard in the first quarter of 2018 using the retrospective approach noted in (ii) above. The Company concluded that its collaboration agreements with Regeneron and Editas will be impacted by the adoption of the new revenue standards. The Company is in the process of allocating the transaction price to the performance obligations in each of the contracts. Preliminarily, the Company anticipates a material impact to its accounting policies, business processes, internal controls and disclosures . In January 2016, the FASB issued ASU No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities In February 2016, the FASB issued ASU No. 2016-2, Leases In June 2016, the FASB issued ASU No. 2016-13 Measurement of Credit Losses on Financial Instruments In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash In May 2017, the FASB issued ASU No. 2017-09, Scope of Modification Accounting The Company has reviewed other recent accounting pronouncements and concluded they are either not applicable to the business or no material effect is expected on the consolidated financial statements as a result of future adoption.

Acquisition of Annapurna (Table

Acquisition of Annapurna (Tables)	12 Months Ended
	Dec. 31, 2017
Business Combinations [Abstract]
Summary of Total Purchase Price Consideration Estimated	Total purchase price consideration was estimated as follows (in thousands): Fair value of common shares issued $ 58,321 Fair value of the Company's common share options exchanged for Annapurna stock options and other stock awards attributable to pre-combination services 7,422 Less: value of common stock and accelerated vesting of stock options at close date (898 ) Total purchase price consideration $ 64,845
Components of Allocation of Total Purchase Price Consideration	The allocation of total purchase price consideration is as follows (in thousands): Cash $ 3,449 Prepaid expenses and other assets 865 Property and equipment 185 Acquired intangible assets 16,200 Goodwill 49,514 Accounts payable (1,118 ) Accrued liabilities (1,848 ) Other noncurrent liabilities (377 ) Deferred tax liabilities (2,025 ) Total purchase price allocation $ 64,845
Summary of Identifiable Intangible Assets Acquired	The identifiable intangible assets acquired consist of IPR&D assets related to products in development, as summarized in the table below (in thousands): Alpha-1 antitrypsin deficiency ("A1AT"),or ADVM-043 $ 11,700 Hereditary angioedema ("HAE"), or ADVM-053 4,500 Total acquired IPR&D intangible assets $ 16,200
Schedule of Pro Forma Financial Information	The following table presents the unaudited pro forma combined results of operations (in thousands, except per share data). Years ended December 31, 2016 2015 Pro forma information Collaboration and license revenue $ 1,455 $ 2,319 Net loss $ (117,551 ) $ (61,774 ) Basic and diluted loss per share $ (2.85 ) $ (1.56 ) Weighted-average common shares outstanding - basic and diluted 41,288 39,566

Restructuring Charges (Tables)

Restructuring Charges (Tables)	12 Months Ended
	Dec. 31, 2017
Restructuring And Related Activities [Abstract]
Summary of Restructuring Activities	The following table summarizes the restructuring activities (in thousands): One-Time Termination Benefits Non-Cash Charge Related to Acceleration of RSUs Total Restructuring liability as of December 31, 2014 $ — $ — $ — Costs incurred and recorded as restructuring charges 1,524 1,049 2,573 Cash payments (511 ) — (511 ) Non-cash settlements — (1,049 ) (1,049 ) Restructuring liability as of December 31, 2015 1,013 — 1,013 Cash payments (988 ) — (988 ) Restructuring liability as of December 31, 2016 25 — 25 Cash payments (25 ) — (25 ) Restructuring liability as of December 31, 2017 $ — $ — $ —

Cash Equivalents and Short-Te29

Cash Equivalents and Short-Term Investments (Tables)	12 Months Ended
	Dec. 31, 2017
Cash And Cash Equivalents [Abstract]
Summary of Cash Equivalents and Short-term Investments	The following is a summary of the Company’s cash equivalents and short-term investments (in thousands): December 31, 2017 Amortized Cost Basis Unrealized Gains Unrealized Loses Estimated Fair Value Money market funds $ 65 $ — $ — $ 65 U.S. government and agency securities 58,351 — (145 ) 58,206 Commercial paper 71,427 — — 71,427 Corporate bonds 38,354 1 (38 ) 38,317 Certificates of deposit 9,731 — — 9,731 Total cash equivalents and short-term investments 177,928 1 (183 ) 177,746 Less: Cash equivalents (57,780 ) — — (57,780 ) Total short-term investments $ 120,148 $ 1 $ (183 ) $ 119,966 December 31, 2016 Money market funds - cash equivalent $ 215,916 $ — $ — $ 215,916 Total cash equivalents $ 215,916 $ — $ — $ 215,916
Summary of Cost and Estimated Fair Value of Marketable Securities Based on Stated Effective Maturities	The following table is a summary of the cost and estimated fair value of the Company’s marketable securities based on stated effective maturities as of December 31, 2017: Amortized Estimated Cost Basis Fair Value (In thousands) Mature within one year $ 169,909 $ 169,758 Mature after one year to three years 8,019 7,988 Total cash equivalents and short-term investments $ 177,928 $ 177,746

Fair Value Measurements and F30

Fair Value Measurements and Fair Value of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Financial Assets and Liabilities Measured and Recognized at Fair Value	The following table summarizes, for assets and liabilities recorded at fair value, the respective fair value and the classification by level of input within the fair value hierarchy as described above (in thousands): Quoted Prices Significant Other Significant Total In Observable Inputs Unobservable Inputs Carrying (Level 1) (Level 2) (Level 3) December 31, 2017 Assets: Money market funds $ 65 $ 65 $ — $ — U.S. government and agency securities 58,206 — 58,206 — Commercial paper 71,427 — 71,427 — Corporate bonds 38,317 38,317 Certificates of deposit 9,731 — 9,731 — Total cash equivalents and short-term investments $ 177,746 $ 65 $ 177,681 $ — Other noncurrent liability: Financing arrangement $ 157 $ — $ — $ 157 December 31, 2016 Assets: Money market funds $ 215,916 $ 215,916 $ — $ — Total cash equivalents $ 215,916 $ 215,916 $ — $ — Other noncurrent liability: Financing arrangement $ 74 $ — $ — $ 74
Quantitative Information About Inputs and Valuation Methodologies Used for Fair Value Measurements	The following table presents quantitative information about the inputs and valuation methodologies used for the fair value measurements classified in Level 3 of the fair value hierarchy: Weighted-Average (range - if applicable) Fair Value at December 31, Valuation Significant Unobservable as of December 31, 2017 2016 Methodology Input 2017 2016 (In thousands) TAP financing $ 157 $ 74 Expected value approach Milestone dates: 2018 - 2023 2017 - 2023 Discount rate: 5.8% 5.5% Percent probability of milestone achievements: 18.2% to 65.0% 18.2% to 80.0%

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consists of the following: December 31, 2017 2016 (In thousands) Computer equipment and software $ 535 $ 300 Laboratory equipment 4,956 4,285 Furniture and fixtures 552 552 Leasehold improvements 1,549 1,522 Construction in progress 105 104 Total property and equipment 7,697 6,763 Less accumulated depreciation and amortization (4,673 ) (2,594 ) Property and equipment, net $ 3,024 $ 4,169

Accrued Expenses and Other Cu32

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2017
Payables And Accruals [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consist of the following: December 31, 2017 2016 (In thousands) Employees' compensation $ 2,259 $ 2,570 Accrued preclinical costs 1,255 1,683 Accrued professional fees 2,295 894 Accrued clinical and process development costs 910 1,142 Other 245 187 Total accrued expenses and other current liabilities $ 6,964 $ 6,476

Financing Arrangements (Tables)

Financing Arrangements (Tables)	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Schedule of TAP Financing Activity	The following table presents the TAP financing activity: Years ended December 31, 2017 2016 (In thousands) Balance of TAP financing liability as of the beginning of the year (1) $ 74 $ — Funding (2) 100 100 Fair value of common stock warrant issued (see Note 12) (3) — (26 ) Gain on fair value of TAP financing liability (4) (17 ) — Balance of TAP financing liability as of the end of the year $ 157 $ 74

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Schedule of Future Minimum Commitments under Facility Operating Lease	As of December 31, 2017, future minimum commitments under the Company’s facility operating lease were as follows: Years ended December 31, Future Commitments (In thousands) 2018 $ 1,162 2019 1,197 2020 403 Total minimum lease payments $ 2,762

Stock Plans (Tables)

Stock Plans (Tables)	12 Months Ended
	Dec. 31, 2017
Schedule of Fair Value of Option Issued to Employees Valuation Assumptions	The fair value of each stock option issued to employees was estimated at the date of grant using the Black-Scholes valuation model with the following weighted-average assumptions: Options Employee Stock Purchase Plan Years ended December 31, Years ended December 31, 2017 2016 2015 2017 2016 2015 Expected volatility 82 % 81 % 79 % 52 % 70 % 74 % Expected term (in years) 6.0 6.1 6.1 0.5 0.5 0.5 Expected dividend yield — — — — — — Risk-free interest rate 1.9 % 1.8 % 1.7 % 1.3 % 0.6 % 0.1 %
Summary of Restricted Stock Units Activity	The following table summarizes the RSU activity under the Company’s stock plans and related information: (In thousands, except grant date fair value and years) Number of Units (in thousands) Weighted- Average Grant Date Fair Value (in dollars) Weighted- Average Remaining Contractual Term (in years) Balance at December 31, 2016 1,049 $ 5.47 1.7 Granted 2,543 2.81 Vested and released (307 ) 6.43 Forfeited (770 ) 3.58 Balance at December 31, 2017 2,515 $ 3.24 1.6
Stock-Based Compensation Expense	The following table presents, by operating expense, the Company’s stock-based compensation expense: Years ended December 31, 2017 2016 2015 (In thousands) Research and development $ 5,253 $ 6,616 $ 4,009 General and administrative 3,470 4,800 6,447 Restructuring charges — — 1,049 Total share-based compensation expense $ 8,723 $ 11,416 $ 11,505
Non Employee Stock Option [Member]
Schedule of Non-Employee Stock Option Plan Valuation Assumptions	The following table presents the weighted-average assumptions used in the Black-Scholes valuation model to estimate the fair value of non-employee stock options: Years ended December 31, 2017 2016 2015 Expected volatility 84 % 82 % 81 % Expected term (in years) 8.8 9.1 3.3 Expected dividend yield — — — Risk-free interest rate 2.3 % 2.1 % 1.0 %
2006 and 2014 Equity Incentive Plan [Member]
Summary of Stock Options Activity	The following table summarizes stock option activity under the Company’s stock plans and related information: (In thousands, except exercise prices and years) Options Outstanding Weighted- Average Exercise Price Weighted- Average Remaining Contract Life (in years) Aggregate Intrinsic Value (a) Balance at December 31, 2016 7,449 $ 4.46 8.4 $ 11,837 Granted 1,989 2.85 Exercised (1,808 ) 0.20 Cancelled/forfeited (935 ) 8.87 Balance at December 31, 2017 6,695 $ 4.51 7.4 $ 9,539 Vested and expected to vest as of December 31, 2017 6,695 $ 4.51 7.4 $ 9,539 Exercisable at December 31, 2017 3,202 $ 4.96 6.1 $ 6,207 (a) The aggregate intrinsic value is calculated as the difference between the stock option exercise price and the closing price of the Company’s common stock of $3.5 per share as of December 31, 2017.

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Schedule of Components of Loss Before Income Taxes	The following table presents domestic and foreign components of loss before provision for income taxes: Years ended December 31, 2017 2016 2015 (In thousands) U.S. $ (36,923 ) $ (96,498 ) $ (47,235 ) Foreign (19,224 ) (18,024 ) (218 ) Loss before income taxes $ (56,147 ) $ (114,522 ) $ (47,453 )
Schedule of Reconciliation of Income Tax Expense Computed Statutory Federal Income Tax and Financial Statements	A reconciliation of income tax expense computed at the statutory federal income tax rate of 34% to income taxes as reflected in the financial statements is as follows: Years ended December 31, 2017 2016 2015 (In thousands) Federal income tax expense at statutory rate $ (19,090 ) $ (38,938 ) $ (16,134 ) Non-deductible foreign research expenses 19 21 45 Stock-based compensation (76 ) 1,356 1,418 Non-deductible expenses 61 985 295 Goodwill impairment — 16,834 — Research and development tax credits (507 ) (326 ) (910 ) Change in valuation allowance 7,935 16,062 15,277 Foreign rate differential 1,495 3,231 9 Rate change 10,163 — — Total tax benefit $ — $ (775 ) $ —
Schedule of Deferred Tax Assets	The following table presents significant components of the Company’s deferred tax assets: As of December 31, 2017 2016 2015 (In thousands) Deferred tax assets: Net operating loss carryforwards $ 27,142 $ 15,785 $ 19,889 Accruals, reserve and other 2,586 3,071 2,681 Stock-based compensation 3,623 3,848 4,328 Tax credit carryforwards 1,774 574 1,945 Property and equipment 274 54 — Intangibles 64 68 45 Total deferred tax assets before valuation allowance 35,463 23,400 28,888 Valuation allowance (35,463 ) (23,400 ) (28,839 ) Total deferred tax assets — — 49 Deferred tax liabilities: Property and equipment — — (49 ) IPR&D (1,250 ) (1,250 ) — Total deferred tax liabilities $ (1,250 ) $ — $ (49 ) Net deferred tax assets $ — $ — $ —
Schedule of Unrecognized Tax Benefits	A reconciliation of the unrecognized tax benefits is as follows: Years ended December 31, 2017 2016 2015 (In thousands) Unrecognized tax benefits as of the beginning of the year $ 2,157 $ 1,568 $ 471 Increase (decrease) related to prior year tax provisions — — 172 Increase related to current year tax provisions 588 589 925 Unrecognized tax benefits as of the end of the year $ 2,745 $ 2,157 $ 1,568

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Schedule of Antidilutive Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share	The following common stock equivalents outstanding at the end of the periods presented were excluded from the calculation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect: As of December 31, 2017 2016 2015 (In thousands) Stock options 6,695 7,449 5,494 Restricted stock units 2,515 1,049 632 ESPP 71 47 42 Warrants to purchase common stock 90 50 40 9,371 8,595 6,208

Selected Quarterly Financial 38

Selected Quarterly Financial Information (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Consolidated Results of Operations	The Company’s quarterly consolidated results of operations are shown below: Quarterly Results of Operations Three Months Ended March 31, 2017 June 30, 2017 September 30, 2017 December 31, 2017 (In thousands, except per share amounts) Revenue $ 462 $ 463 $ 463 $ 461 Total operating expenses (1) (17,050 ) (12,556 ) (15,034 ) (16,056 ) Net loss (16,099 ) (11,430 ) (13,829 ) (14,789 ) Basic and diluted net loss per share (0.38 ) (0.27 ) (0.32 ) (0.32 ) Quarterly Results of Operations Three Months Ended March 31, 2016 June 30, 2016 September 30, 2016 December 31, 2016 (In thousands, except per share amounts) Revenue $ 265 $ 307 $ 395 $ 488 Total operating expenses (2)(3)(4) (15,773 ) (62,189 ) (14,902 ) (23,875 ) Net loss (15,392 ) (61,660 ) (14,301 ) (22,394 ) Basic and diluted net loss per share (0.57 ) (1.76 ) (0.35 ) (0.54 ) (1) During the year ended December 31, 2017, the Company recorded a total of $2.0 million of estimated costs associated with the termination of MSA with Cornell University (see Note 7). During the three months ended March 31, 2017, the Company recorded a total of $2.3 million of these estimated costs, which was subsequently adjusted by $0.3 million during the three months ended June 30, 2017. (2) During the year ended December 31, 2016, the Company performed a two-step goodwill impairment analysis and recorded $49.1 million and $0.4 million of goodwill impairment charge, respectively, during the three months ended June and September, 2016. ( 3 ) During the three months ended December 31, 2016, the Company performed its annual assessment of its intangible assets, ADVM-043 and ADVM-053, and recorded a total of $11.2 million of impairment charge related to its intangible assets. ( 4 ) During the three months ended March 31, 2016, two officers of the company resigned and the Company recorded $2.2 million of one-time stock-based compensation charge related to the accelerated vesting of their stock-based awards.

Description of the Business - A

Description of the Business - Additional Information (Detail) - USD ($)	1 Months Ended					12 Months Ended
	Feb. 28, 2018	Jan. 31, 2018	Aug. 31, 2017	Mar. 31, 2015	Jan. 31, 2015	Dec. 31, 2017	Dec. 31, 2015	Dec. 31, 2016
Class of Stock [Line Items]
Date of incorporation						Jul. 17, 2006
Change of entity name date						May 11, 2016
Accumulated deficit						$ (254,062,000)		$ (197,915,000)
Net proceeds from issuance of common stock				$ 8,300,000		16,519,000	$ 138,954,000
Issuance of common stock				230,000
IPO and Private Placement and Follow On Offering [Member]
Class of Stock [Line Items]
Net proceeds from issuance of common stock					$ 237,100,000
2017 Stock Offering Agreement [Member]
Class of Stock [Line Items]
Net proceeds from issuance of common stock						$ 16,500,000
Issuance of common stock						5,130,339
2017 Stock Offering Agreement [Member] | Maximum [Member]
Class of Stock [Line Items]
Aggregate offering price of common stock			$ 50,000,000
2017 Stock Offering Agreement [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Net proceeds from issuance of common stock		$ 5,700,000
Issuance of common stock		1,419,893
Underwritten Public Offering [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Net proceeds from issuance of common stock	$ 64,300,000
Issuance of common stock	10,222,235

Summary of Significant Accoun40

Summary of Significant Accounting Policies - Additional Information (Detail)

1 Months Ended

3 Months Ended

12 Months Ended

Feb. 28, 2018USD ($)

Aug. 31, 2016USD ($)

May 31, 2014USD ($)

Dec. 31, 2017USD ($)

Sep. 30, 2017USD ($)

Jun. 30, 2017USD ($)

Mar. 31, 2017USD ($)

Dec. 31, 2016USD ($)

Sep. 30, 2016USD ($)

Jun. 30, 2016USD ($)

Mar. 31, 2016USD ($)

Dec. 31, 2017USD ($)Segment

Dec. 31, 2016USD ($)

Dec. 31, 2015USD ($)

Dec. 31, 2014USD ($)

Summary Of Significant Accounting Policies [Line Items]

Number of reportable segments | Segment

1

Number of operating segments | Segment

1

Impairment losses of amortizable long-lived assets

$ 0

$ 0

Impairment of intangible assets

$ 11,200,000

Collaboration and license revenue

$ 461,000

$ 463,000

$ 463,000

$ 462,000

$ 488,000

$ 395,000

$ 307,000

$ 265,000

1,849,000

1,455,000

$ 2,319,000

Tax credits due to reduction of research and development expense

507,000

326,000

910,000

Australian Tax Authorities [Member]

Summary Of Significant Accounting Policies [Line Items]

Tax credits due to reduction of research and development expense

100,000

300,000

100,000

Upfront Payment Arrangement [Member]

Summary Of Significant Accounting Policies [Line Items]

Initial payments received

$ 8,000,000

Editas Medicine Inc [Member]

Summary Of Significant Accounting Policies [Line Items]

Initial payments received

1,000,000

Revenue recognized

$ 1,000,000

Performance period

3 years

Collaboration and license revenue

300,000

100,000

Editas Medicine Inc [Member] | Subsequent Event [Member]

Summary Of Significant Accounting Policies [Line Items]

One-time, non-refundable cash payments from collaborators

$ 500,000

Regeneron Corporation [Member]

Summary Of Significant Accounting Policies [Line Items]

Collaboration and license revenue

$ 1,500,000

1,400,000

800,000

Regeneron Corporation [Member] | Upfront Payment Arrangement [Member]

Summary Of Significant Accounting Policies [Line Items]

Revenue recognized

$ 6,500,000

$ 1,500,000

Performance period

8 years

IPR&D [Member]

Summary Of Significant Accounting Policies [Line Items]

Impairment of intangible assets

11,200,000

Research Services [Member] | Editas Medicine Inc [Member]

Summary Of Significant Accounting Policies [Line Items]

Initial payments received

$ 500,000

Minimum [Member]

Summary Of Significant Accounting Policies [Line Items]

Estimated useful lives of assets

3 years

Maximum [Member]

Summary Of Significant Accounting Policies [Line Items]

Estimated useful lives of assets

5 years

Acquisition of Annapurna - Addi

Acquisition of Annapurna - Additional Information (Detail)	May 11, 2016USD ($)Segment$ / sharesshares	Dec. 31, 2016USD ($)	Jun. 30, 2016USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2017	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Business Acquisition [Line Items]
Total purchase price consideration						$ 64,845,000
Impairment of intangible assets		$ 11,200,000
IPR&D [Member]
Business Acquisition [Line Items]
Impairment of intangible assets						$ 11,200,000
Stock Options [Member]
Business Acquisition [Line Items]
Expected volatility					82.00%	81.00%	79.00%
Risk-free interest rate					1.90%	1.80%	1.70%
Expected term (in years)					6 years	6 years 1 month 6 days	6 years 1 month 6 days
Non Employee Stock Option [Member]
Business Acquisition [Line Items]
Expected volatility					84.00%	82.00%	81.00%
Risk-free interest rate					2.30%	2.10%	1.00%
Expected term (in years)					8 years 9 months 18 days	9 years 1 month 6 days	3 years 3 months 18 days
Annapurna Therapeutics SAS [Member]
Business Acquisition [Line Items]
Date of acquisition					May 11, 2016
Total purchase price consideration	$ 64,845,000
Fair value of stock awards related to accelerated vesting of stock options and common shares	$ 900,000
Common stock exchange ratio					9.54655
Goodwill, description					Goodwill represents expected synergies of two combined companies.
Acquisition costs						$ 2,500,000
Fair value, discounted rate	11.00%
Goodwill	$ 49,514,000		$ 0
Number of reporting unit | Segment	1
Amount of net loss included in the consolidated statements of operations from acquisition date				$ 16,600,000
Revenues from the acquisition date				0
Interest expense related to convertible notes and changes in fair value of preferred stock warrants						1,000,000	$ 500,000
Bonuses paid	$ 400,000
Annapurna Therapeutics SAS [Member] | IPR&D [Member]
Business Acquisition [Line Items]
Impairment of intangible assets				$ 11,200,000
Annapurna Therapeutics SAS [Member] | Stock Options [Member]
Business Acquisition [Line Items]
Shares reserved for future exercise of stock options | shares	3,673,940
Fair value of estimated stock awards at acquisition	$ 14,700,000
Dividends	0.00%
Expected volatility	80.00%
Risk-free interest rate	1.40%
Expected term (in years)	6 years
Fair value of stock options attributed as pre-combination service and included as part of acquisition consideration	$ 7,400,000
Fair value of stock options post-combination attribution recognized as compensation expense	$ 7,200,000
Stock-based compensation expense related to vesting of options			$ 900,000			900,000
Annapurna Therapeutics SAS [Member] | Stock Options [Member] | Executives [Member]
Business Acquisition [Line Items]
Stock-based compensation expense related to vesting of options						$ 200,000	$ 400,000
Annapurna Therapeutics SAS [Member] | Non Employee Stock Option [Member]
Business Acquisition [Line Items]
Dividends	0.00%
Expected volatility	89.00%
Risk-free interest rate	1.10%
Expected term (in years)	10 years
Annapurna Therapeutics SAS [Member] | Common Shares [Member]
Business Acquisition [Line Items]
Number of shares issued to acquired an entity | shares	14,087,246
Share price of common stock on date of acquisition | $ / shares	$ 4.14

Acquisition of Annapurna - Summ

Acquisition of Annapurna - Summary of Total Purchase Price Consideration Estimated (Detail) - USD ($) $ in Thousands	May 11, 2016	Dec. 31, 2016
Business Acquisition [Line Items]
Total purchase price consideration		$ 64,845
Annapurna Therapeutics SAS [Member]
Business Acquisition [Line Items]
Less: value of common stock and accelerated vesting of stock options at close date	$ (898)
Total purchase price consideration	64,845
Annapurna Therapeutics SAS [Member] | Common Shares [Member]
Business Acquisition [Line Items]
Fair value of common shares issued/common share options exchanged	58,321
Annapurna Therapeutics SAS [Member] | Common Shares [Member] | Stock Options [Member]
Business Acquisition [Line Items]
Fair value of common shares issued/common share options exchanged	$ 7,422

Acquisition of Annapurna - Comp

Acquisition of Annapurna - Components of Allocation of Total Purchase Price Consideration (Detail) - Annapurna Therapeutics SAS [Member] - USD ($) $ in Thousands	Jun. 30, 2016	May 11, 2016
Business Acquisition [Line Items]
Cash		$ 3,449
Prepaid expenses and other assets		865
Property and equipment		185
Acquired intangible assets		16,200
Goodwill	$ 0	49,514
Accounts payable		(1,118)
Accrued liabilities		(1,848)
Other noncurrent liabilities		(377)
Deferred tax liabilities		(2,025)
Total purchase price allocation		$ 64,845

Acquisition of Annapurna - Su44

Acquisition of Annapurna - Summary of Identifiable Intangible Assets Acquired (Detail) - Annapurna Therapeutics SAS [Member] $ in Thousands	May 11, 2016USD ($)
Acquired Indefinite Lived Intangible Assets [Line Items]
Acquired intangible assets	$ 16,200
IPR&D [Member]
Acquired Indefinite Lived Intangible Assets [Line Items]
Acquired intangible assets	16,200
IPR&D [Member] | Alpha-1 Antitrypsin Deficiency [Member]
Acquired Indefinite Lived Intangible Assets [Line Items]
Acquired intangible assets	11,700
IPR&D [Member] | Hereditary Angioedema [Member]
Acquired Indefinite Lived Intangible Assets [Line Items]
Acquired intangible assets	$ 4,500

Acquisition of Annapurna - Sche

Acquisition of Annapurna - Schedule of Pro Forma Financial Information (Detail) - Annapurna Therapeutics SAS [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Pro forma information
Collaboration and license revenue	$ 1,455	$ 2,319
Net loss	$ (117,551)	$ (61,774)
Basic and diluted loss per share	$ (2.85)	$ (1.56)
Weighted-average common shares outstanding - basic and diluted	41,288	39,566

Acquisition of Annapurna - Ad46

Acquisition of Annapurna - Additional Information1 (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2017	May 11, 2016
Business Acquisition [Line Items]
Common shares outstanding			41,805,009	49,015,339
Goodwill impairment charge	$ 400	$ 49,100
Impairment of goodwill and intangible assets			$ 60,714
Annapurna Therapeutics SAS [Member]
Business Acquisition [Line Items]
Stock price		$ 3.16
Common shares outstanding		41,300,000
Goodwill		$ 0			$ 49,514
Annapurna Therapeutics SAS [Member] | Alpha-1 Antitrypsin Deficiency and Hereditary Angioedema [Member] | IPR&D [Member]
Business Acquisition [Line Items]
Carrying value of intangible assets			16,200
Fair value of intangible assets			5,000
Impairment of goodwill and intangible assets			11,200
Annapurna Therapeutics SAS [Member] | Impairment of Goodwill and Intangible Assets [Member]
Business Acquisition [Line Items]
Goodwill impairment charge			$ 49,500

Restructuring Charges - Additio

Restructuring Charges - Additional Information (Detail) $ in Thousands	Dec. 22, 2015Employee	Mar. 31, 2016USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Restructuring And Related Cost [Line Items]
Company workforce reduction percentage under restructuring plan	20.00%
Number of employees eliminated | Employee	15
Restructuring charges					$ 2,573
Stock-based compensation expense		$ 2,200	$ 8,723	$ 11,416	11,505
RSUs | Restricted Stock Units [Member]
Restructuring And Related Cost [Line Items]
Stock-based compensation expense					$ 1,000

Restructuring Charges - Summary

Restructuring Charges - Summary of Restructuring Activities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring Reserve [Line Items]
Restructuring liability beginning balance	$ 25	$ 1,013	$ 0
Costs incurred and recorded as restructuring charges			2,573
Cash payments	(25)	(988)	(511)
Non-cash settlements			(1,049)
Restructuring liability ending balance	0	25	1,013
One-Time Termination Benefits [Member]
Restructuring Reserve [Line Items]
Restructuring liability beginning balance	25	1,013	0
Costs incurred and recorded as restructuring charges			1,524
Cash payments	(25)	(988)	(511)
Restructuring liability ending balance	0	25	1,013
Non-Cash Charge Related to Acceleration of RSUs [Member]
Restructuring Reserve [Line Items]
Restructuring liability beginning balance	0	0	0
Costs incurred and recorded as restructuring charges			1,049
Non-cash settlements			(1,049)
Restructuring liability ending balance	$ 0	$ 0	$ 0

Cash Equivalents and Short-Te49

Cash Equivalents and Short-Term Investments - Summary of Cash Equivalents and Short-term Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	$ 120,148
Unrealized Gains	1
Unrealized Loses	(183)
Estimated Fair Value	119,966
Less: Cash Equivalents, Amortized Cost Basis	(57,780)
Less: Cash Equivalents, Unrealized Gains	0
Less: Cash Equivalents, Unrealized Loses	0
Less: Cash Equivalents, Estimated Fair Value	(57,780)
Money Market Funds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	65	$ 215,916
Estimated Fair Value	65	215,916
U.S. Government Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	58,351
Unrealized Loses	(145)
Estimated Fair Value	58,206
Commercial Paper [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	71,427
Estimated Fair Value	71,427
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	9,731
Estimated Fair Value	9,731
Total Cash Equivalents [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis		215,916
Estimated Fair Value		$ 215,916
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	38,354
Unrealized Gains	1
Unrealized Loses	(38)
Estimated Fair Value	38,317
Total Cash Equivalents and Short-term Investments [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	177,928
Unrealized Gains	1
Unrealized Loses	(183)
Estimated Fair Value	$ 177,746

Cash Equivalents and Short-Te50

Cash Equivalents and Short-Term Investments - Additional Information (Detail)	Dec. 31, 2017USD ($)
Available For Sale Securities [Abstract]
Marketable securities other-than-temporarily impaired	$ 0

Cash Equivalents and Short-Te51

Cash Equivalents and Short-Term Investments - Summary of Cost and Estimated Fair Value of Marketable Securities Based on Stated Effective Maturities (Detail) $ in Thousands	Dec. 31, 2017USD ($)
Investments Debt And Equity Securities [Abstract]
Amortized Cost Basis, Mature within one year	$ 169,909
Amortized Cost Basis, Mature after one year to three years	8,019
Amortized Cost Basis, Total cash equivalents and short-term investments	177,928
Estimated Fair Value, Mature within one year	169,758
Estimated Fair Value, Mature after one year to three years	7,988
Estimated Fair Value, Total cash equivalents and short-term investments	$ 177,746

Fair Value Measurements and F52

Fair Value Measurements and Fair Value of Financial Instruments - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Aug. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities transferred within Level 3	$ 0	$ 0
The Alpha-1 Project, Inc. [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amount borrowed under financing arrangement			$ 300,000
Proceeds from financing arrangement	$ 157,000	$ 74,000

Fair Value Measurements and F53

Fair Value Measurements and Fair Value of Financial Instruments - Financial Assets and Liabilities Measured and Recognized at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets:
Assets	$ 177,746	$ 215,916
Corporate Bonds [Member]
Assets:
Assets	38,317
Financing Arrangement [Member]
Other noncurrent liability:
Other noncurrent liability	157	74
Money Market Funds [Member]
Assets:
Assets	65	215,916
Commercial Paper [Member]
Assets:
Assets	71,427
Certificates of Deposit [Member]
Assets:
Assets	9,731
U.S. Government and Agency Securities [Member]
Assets:
Assets	58,206
Quoted Prices in Active Markets (Level 1) [Member]
Assets:
Assets	65	215,916
Quoted Prices in Active Markets (Level 1) [Member] | Money Market Funds [Member]
Assets:
Assets	65	215,916
Significant Other Observable Inputs (Level 2) [Member]
Assets:
Assets	177,681
Significant Other Observable Inputs (Level 2) [Member] | Corporate Bonds [Member]
Assets:
Assets	38,317
Significant Other Observable Inputs (Level 2) [Member] | Commercial Paper [Member]
Assets:
Assets	71,427
Significant Other Observable Inputs (Level 2) [Member] | Certificates of Deposit [Member]
Assets:
Assets	9,731
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Agency Securities [Member]
Assets:
Assets	58,206
Significant Unobservable Inputs (Level 3) [Member] | Financing Arrangement [Member]
Other noncurrent liability:
Other noncurrent liability	$ 157	$ 74

Fair Value Measurements and F54

Fair Value Measurements and Fair Value of Financial Instruments - Quantitative Information About Inputs and Valuation Methodologies Used for Fair Value Measurements (Detail) - Significant Unobservable Inputs (Level 3) [Member] - The Alpha-1 Project, Inc. [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other noncurrent liability	$ 157	$ 74
Valuation Methodology	Expected value approach
Fair value, discounted rate	5.80%	5.50%
Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone dates	2,018	2,017
Percent probability of milestone achievements	18.20%	18.20%
Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone dates	2,023	2,023
Percent probability of milestone achievements	65.00%	80.00%

Significant Agreements - Additi

Significant Agreements - Additional Information (Detail) € in Millions

1 Months Ended

3 Months Ended

12 Months Ended

Feb. 28, 2018USD ($)

Jan. 31, 2018USD ($)

Feb. 28, 2017

Dec. 31, 2016Agreement

Aug. 31, 2016USD ($)

Dec. 31, 2015Agreement

Oct. 31, 2015USD ($)

Oct. 31, 2015EUR (€)

May 31, 2014USD ($)Target

Apr. 30, 2014USD ($)

May 31, 2010USD ($)

Mar. 31, 2017USD ($)

Dec. 31, 2017USD ($)

Dec. 31, 2017EUR (€)

Dec. 31, 2016USD ($)

Dec. 31, 2015USD ($)

Jan. 25, 2018USD ($)

License Agreement [Line Items]

Research and development

$ 39,839,000

$ 31,670,000

$ 25,462,000

Annual maintenance fee payable

$ 500,000

$ 600,000

Regeneron Corporation [Member]

License Agreement [Line Items]

Agreement termination scenario description

Regeneron may terminate the agreement for convenience at any time on a target by target basis or in totality upon a 30-day notice. Each party can terminate the agreement if another party commits a material breach or material default in performance of its obligations and such breach or default is not cured within 60 days. The agreement is automatically terminated upon initiation of any bankruptcy proceedings, reorganization or dissolution of either party. The Company can terminate the agreement upon 30-day notice if Regeneron challenges the validity, scope or enforceability of any Company patent.

Regeneron may terminate the agreement for convenience at any time on a target by target basis or in totality upon a 30-day notice. Each party can terminate the agreement if another party commits a material breach or material default in performance of its obligations and such breach or default is not cured within 60 days. The agreement is automatically terminated upon initiation of any bankruptcy proceedings, reorganization or dissolution of either party. The Company can terminate the agreement upon 30-day notice if Regeneron challenges the validity, scope or enforceability of any Company patent.

Potential target of collaborations | Target

8

Initial collaboration term

3 years

Additional collaboration term

3 years

Collaboration agreement expiration date

May 1, 2020

Collaboration fee for exclusive worldwide license per target

$ 2,000,000

Potential milestone earnings

640,000,000

Reimbursement of research and development expense

35.00%

35.00%

Regeneron Corporation [Member] | Development and Regulatory Milestone [Member]

License Agreement [Line Items]

Potential milestone earnings

$ 80,000,000

University of California [Member]

License Agreement [Line Items]

Aggregate amount of milestone payments for one indication

$ 900,000

Additional milestone payments recognized

$ 0

Agreement termination scenario description

The Company may terminate this agreement without cause at any time upon 30 days’ prior written notice to the Regents. The Regents may terminate this agreement for a breach by the Company that remains uncured for 60 days, if the Company becomes insolvent, if the Company directly or through a third party files a claim that a licensed patent right is invalid or unenforceable, or if the Company fails to meet or extend the date for meeting certain diligence milestones.

License agreement termination notice period

30 days

University of California [Member] | Maximum [Member]

License Agreement [Line Items]

Additional milestone payments for up to two additional indications

$ 500,000

Editas Medicine Inc [Member]

License Agreement [Line Items]

Non-refundable upfront payment received

$ 1,000,000

Non-refundable upfront payment, obligation to fund research and development costs

500,000

Collaboration, option and license agreement fee receivable per indication

$ 1,000,000

Range of tiered royalties receivable on net sales

between the mid-single digits and low teens

between the mid-single digits and low teens

Editas Medicine Inc [Member] | Initial Indication [Member]

License Agreement [Line Items]

Collaboration, option and license agreement fee receivable per indication

1,300

Editas Medicine Inc [Member] | Additional Indication [Member]

License Agreement [Line Items]

Collaboration, option and license agreement fee receivable per indication

1,500

Editas Medicine Inc [Member] | Subsequent Event [Member]

License Agreement [Line Items]

Initial payments received

$ 500,000

$ 500,000

Collaboration, option and license agreement fee receivable per indication

$ 1,000,000

Editas Medicine Inc [Member] | Maximum [Member]

License Agreement [Line Items]

Potential milestone earnings

$ 15,500,000

Cornell University [Member]

License Agreement [Line Items]

Additional milestone payments recognized

$ 0

0

Research and development

$ 1,800,000

Agreement termination effective date

Jan. 6, 2017

Jan. 6, 2017

Number Of License Agreements | Agreement

3

3

Aggregate milestone payments and royalties on sales

$ 16,000,000

Cornell University [Member] | Consultant [Member]

License Agreement [Line Items]

Annual compensation

200,000

Cornell University [Member] | Master Service Agreement [Member]

License Agreement [Line Items]

Estimated costs associated with the termination

$ 2,300,000

2,000,000

Cornell University [Member] | Maximum [Member]

License Agreement [Line Items]

Annual maintenance fee payable

30,000

Cornell University [Member] | Minimum [Member]

License Agreement [Line Items]

Annual maintenance fee payable

$ 300,000

REGENXBIO [Member] | A1AT Deficiency/Allergy License Agreement [Member]

License Agreement [Line Items]

License agreement termination notice period

6 months

6 months

Aggregate milestone payments and royalties on sales

$ 20,000,000

License agreement termination effective date

2016-10

2016-10

REGENXBIO [Member] | Friedreich's Ataxia License Agreement [Member]

License Agreement [Line Items]

License agreement termination notice period

6 months

6 months

Aggregate milestone payments and royalties on sales

$ 13,900,000

License agreement termination effective date

2018-04

Inserm Transfert [Member]

License Agreement [Line Items]

Additional milestone payments recognized

$ 0

Agreement termination scenario description

The Company may terminate this agreement upon 60 days’ prior written notice. Inserm may terminate this license agreement if the Company becomes the subject of a voluntary or involuntary petition in bankruptcy or fails to meet development milestones and such failure is not cured within a specified number of days. Inserm may also terminate the license granted to the Company in a given country if the Company (i) before regulatory approval of a product in any country, has ceased conducting any development of products in all countries for 12 consecutive months or (ii) after regulatory approval of a product in a given country, has ceased marketing such product in such country for 12 consecutive months.

The Company may terminate this agreement upon 60 days’ prior written notice. Inserm may terminate this license agreement if the Company becomes the subject of a voluntary or involuntary petition in bankruptcy or fails to meet development milestones and such failure is not cured within a specified number of days. Inserm may also terminate the license granted to the Company in a given country if the Company (i) before regulatory approval of a product in any country, has ceased conducting any development of products in all countries for 12 consecutive months or (ii) after regulatory approval of a product in a given country, has ceased marketing such product in such country for 12 consecutive months.

License agreement termination notice period

60 days

60 days

Aggregate milestone payments and royalties on sales | €

€ 2

One-time payment due to termination of license agreement | €

€ 0.3

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 7,697	$ 6,763
Less accumulated depreciation and amortization	(4,673)	(2,594)
Property and equipment, net	3,024	4,169
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	535	300
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	4,956	4,285
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	552	552
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	1,549	1,522
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 105	$ 104

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 2,096	$ 1,603	$ 812

Accrued Expenses and Other Cu58

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accrued Expenses And Other Current Liabilities [Abstract]
Employees' compensation	$ 2,259	$ 2,570
Accrued preclinical costs	1,255	1,683
Accrued professional fees	2,295	894
Accrued clinical and process development costs	910	1,142
Other	245	187
Total accrued expenses and other current liabilities	$ 6,964	$ 6,476

Financing Arrangements - Additi

Financing Arrangements - Additional Information (Detail)	1 Months Ended			12 Months Ended
	Dec. 31, 2017USD ($)	Sep. 30, 2016USD ($)$ / sharesshares	Jul. 31, 2016USD ($)Tranche$ / sharesshares	Dec. 31, 2017EUR (€)	Dec. 31, 2016USD ($)	Dec. 31, 2016EUR (€)	Aug. 31, 2015EUR (€)
Line Of Credit Facility [Line Items]
Proceeds from financing arrangement					$ 100,000
The Alpha-1 Project, Inc. [Member]
Line Of Credit Facility [Line Items]
Potential milestone earnings	$ 100,000
Number Of Tranches | Tranche			3
Proceeds from financing arrangement		$ 100,000
The Alpha-1 Project, Inc. [Member] | Common Shares [Member]
Line Of Credit Facility [Line Items]
Number of shares issued on exercise of warrants | shares		10,000	10,000
Exercise price | $ / shares		$ 4.33	$ 4.33
Expiration period		5 years	5 years
The Alpha-1 Project, Inc. [Member] | Maximum [Member]
Line Of Credit Facility [Line Items]
Potential milestone earnings			$ 300,000
Expected percentage of milestone repayments			450.00%
BPI France [Member]
Line Of Credit Facility [Line Items]
Interest free conditional advance, maximum borrowing capacity | €							€ 800,000
Interest free conditional advance	$ 400,000				400,000	€ 500,000
Interest free conditional advance, frequency of payments				Quarterly
Interest free conditional advance, periodic payment | €				€ 25,000
Interest free conditional advance, beginning date of periodic payment				Sep. 30, 2017
Interest free conditional advance, ending date of periodic payment				Jun. 30, 2022
Interest free conditional advance, imputed interest rate	7.00%
BPI France [Member] | Other Noncurrent Liabilities [Member]
Line Of Credit Facility [Line Items]
Interest free conditional advance	$ 300,000				300,000
BPI France [Member] | Accrued Expenses and Other Current Liabilities [Member]
Line Of Credit Facility [Line Items]
Interest free conditional advance	$ 100,000				$ 100,000

Financing Arrangements - Summar

Financing Arrangements - Summary of TAP Financing Activity (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Line Of Credit Facility [Line Items]
Balance of TAP financing liability as of the beginning of the year	$ 386
Balance of TAP financing liability as of the end of the year	481	$ 386
TAP [Member]
Line Of Credit Facility [Line Items]
Balance of TAP financing liability as of the beginning of the year	74
Funding	100	100
Fair value of common stock warrant issued (see Note 12)		(26)
Gain on fair value of TAP financing liability	(17)
Balance of TAP financing liability as of the end of the year	$ 157	$ 74

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	Mar. 16, 2017USD ($)	Dec. 31, 2015Claim	Jul. 31, 2015Claim	Mar. 31, 2017USD ($)	Dec. 31, 2017USD ($)Claim	Dec. 31, 2016USD ($)Claim	Dec. 31, 2015USD ($)
Other Commitments [Line Items]
Lease renewal term					4 years
Lease expiration date					May 8, 2020
Rent expense					$ 1,800,000	$ 1,700,000	$ 1,500,000
Contractual obligation for manufacturing					4,200,000
Accrued royalties					0
Annual maintenance fee payable					500,000	600,000
Claims paid to date related to indemnification issues					$ 0	$ 0
Number of claims to date | Claim		1	3		0	0
Accruals or expenses related to indemnification issues					$ 0	$ 0
Proposed aggregate amount of settlement	$ 13,000,000
General and administrative expense					$ 20,857,000	$ 24,355,000	$ 22,107,000
Indemnification Obligations [Member]
Other Commitments [Line Items]
Proposed aggregate amount of settlement	$ 1,000,000
General and administrative expense				$ 1,000,000

Commitments and Contingencies62

Commitments and Contingencies - Schedule of Future Minimum Commitments under Facility Operating Lease (Detail) $ in Thousands	Dec. 31, 2016USD ($)
Commitments And Contingencies Disclosure [Abstract]
2,018	$ 1,162
2,019	1,197
2,020	403
Total minimum lease payments	$ 2,762

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($)	1 Months Ended			12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jul. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Class Of Warrant Or Right [Line Items]
Common stock, shares authorized					300,000,000	300,000,000
Preferred stock, shares authorized					5,000,000	5,000,000
Preferred stock, shares issued					0	0
Preferred stock, shares outstanding					0	0
The Alpha-1 Project, Inc. [Member]
Class Of Warrant Or Right [Line Items]
Fair value of common stock price per share			$ 4.33
Contractual term of warrant			5 years
Fair value of obligation risk free interest rate			1.07%
Fair value of obligation expected volatility rate			72.00%
Fair value of obligation expected dividend yield			0.00%
Value of warrants			$ 26,000
The Alpha-1 Project, Inc. [Member] | Common Shares [Member]
Class Of Warrant Or Right [Line Items]
Warrant to purchase shares of common stock		10,000	10,000
Exercise price		$ 4.33	$ 4.33
Expiration period		5 years	5 years
LEI [Member]
Class Of Warrant Or Right [Line Items]
Warrant to purchase shares of common stock	40,000			40,000
Exercise price	$ 3.65			$ 10.51
Warrant expiration date	Sep. 29, 2022			Oct. 15, 2020
Fair value of obligation to issue warrant	$ 100,000			$ 200,000
Fair value of common stock price per share	$ 3.65			$ 8.35
Contractual term of warrant	5 years			5 years
Fair value of obligation risk free interest rate	1.89%			1.34%
Fair value of obligation expected volatility rate	91.00%			75.00%
Fair value of obligation expected dividend yield	0.00%			0.00%

Stock Plans - Additional Inform

Stock Plans - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended				12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2015	Jul. 31, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Oct. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total intrinsic value of stock options exercised					$ 4.8	$ 7.1	$ 11.2
Weighted-average fair values of options granted					$ 1.99	$ 3.35	$ 15.91
Unrecognized stock-based compensation expense related to employees' awards					$ 8.9
Unrecognized stock-based compensation, weighted-average period					2 years 2 months 12 days
One time share based payment charge related to cancellation of unvested stock option of CEO							$ 2.4
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
One time charges stock-based compensation expense for modifications						$ 1.5
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
One time charges stock-based compensation expense for modifications						$ 1.4
Executives [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock options granted	150,000	518,000
Principal Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock options granted			910,000
Plans [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares authorized for issuance					17,167,856
Shares available for future grants					2,830,010
Non Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted period					8 years 9 months 18 days	9 years 1 month 6 days	3 years 3 months 18 days
Unrecognized stock-based compensation expense related to employees' awards					$ 0.4
Unrecognized stock-based compensation, weighted-average period					1 year 6 months
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock-based compensation, weighted-average period					2 years 10 months 24 days
Weighted-average grant date fair value of RSUs granted					$ 2.81	$ 4.40	$ 13.85
Total fair values of RSUs vested					$ 2	$ 1.7	$ 1.4
Unrecognized compensation cost					$ 6.4
Restricted Stock Units [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units granted to employees vesting period					4 years
Restricted Stock Units [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units granted to employees vesting period					2 years
2014 Equity Incentive Award Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of annual increase in number of shares available for future issuance				4.00%
2017 Inducement Plan [Member] | Stock Options and Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares available for future grant								600,000
2014 Plan And Inducement Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted description					Stock options under the 2014 Plan and the Inducement Plan may be granted for periods of up to 10 years and at prices no less than 100% of the estimated fair value of the shares on the date of grant as determined by the board of directors, provided, however, that the exercise price of an ISO and NSO granted to a 10% stockholder may not be less than 110% of the estimated fair value of the shares on the date of grant.
Stock options granted to employees and non-employees vesting period					4 years
2014 Plan And Inducement Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted period					10 years
2014 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares available for future grant				208,833
Shares available for future grants					962,095
Percentage increase in shares issued				1.00%
Common stock, shares issued					74,642

Stock Plans - Summary of Stock

Stock Plans - Summary of Stock Options Activity (Detail) - 2006 Equity Incentive Plan [Member] - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Options Outstanding [Roll Forward]
Options Outstanding, Beginning Balance	7,449,000
Options Outstanding, Granted	1,989,000
Options Outstanding, Exercised	(1,808,000)
Options Outstanding, Cancelled/forfeited	(935,000)
Options Outstanding, Ending Balance	6,695,000	7,449,000
Options Outstanding, Vested and expected to vest	6,695,000
Options Outstanding, Exercisable	3,202,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-Average Exercise Price, Beginning Balance	$ 4.46
Weighted-Average Exercise Price, Granted	2.85
Weighted-Average Exercise Price, Exercised	0.20
Weighted-Average Exercise Price, Cancelled/forfeited	8.87
Weighted-Average Exercise Price, Ending Balance	4.51	$ 4.46
Weighted-Average Exercise Price, Vested and expected to vest	4.51
Weighted-Average Exercise Price, Exercisable	$ 4.96
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-Average Remaining Contractual Life (In years)	7 years 4 months 24 days	8 years 4 months 24 days
Weighted-Average Remaining Contractual Life (In years), Vested and expected to vest	7 years 4 months 24 days
Weighted-Average Remaining Contractual Life (In years), Exercisable	6 years 1 month 6 days
Aggregate Intrinsic Value, Options outstanding	$ 9,539	$ 11,837
Aggregate Intrinsic Value, Vested and expected to vest	9,539
Aggregate Intrinsic Value, Exercisable	$ 6,207

Stock Plans - Summary of Stoc66

Stock Plans - Summary of Stock Options Activity (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2017$ / shares
Equity [Abstract]
Aggregate intrinsic value of option per share	$ 3.5

Stock Plans - Schedule of Emplo

Stock Plans - Schedule of Employees Stock Purchase Plan Valuation Assumptions (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	82.00%	81.00%	79.00%
Expected term (in years)	6 years	6 years 1 month 6 days	6 years 1 month 6 days
Risk-free interest rate	1.90%	1.80%	1.70%
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	52.00%	70.00%	74.00%
Expected term (in years)	6 months	6 months	6 months
Risk-free interest rate	1.30%	0.60%	0.10%
Non Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	84.00%	82.00%	81.00%
Expected term (in years)	8 years 9 months 18 days	9 years 1 month 6 days	3 years 3 months 18 days
Risk-free interest rate	2.30%	2.10%	1.00%

Stock Plans - Summary of Restri

Stock Plans - Summary of Restricted Stock Units Activity (Detail) - Restricted Stock Units [Member] - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Beginning Balance	1,049,000
Number of Units, Granted	2,543,000
Number of Units, Vested and released	(307,000)
Number of Units, Forfeited	(770,000)
Number of Units, Ending Balance	2,515,000	1,049,000
Weighted-Average Grant-Date Fair Value, Beginning Balance	$ 5.47
Weighted-Average Grant-Date Fair Value, Granted	2.81	$ 4.40	$ 13.85
Weighted-Average Grant-Date Fair Value, Vested and released	6.43
Weighted-Average Grant-Date Fair Value, Forfeited	3.58
Weighted-Average Grant-Date Fair Value, Ending Balance	$ 3.24	$ 5.47
Weighted-Average Remaining Contractual Term (In years)	1 year 7 months 6 days	1 year 8 months 12 days

Stock Plans - Stock-Based Compe

Stock Plans - Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total share-based compensation expense	$ 8,723	$ 11,416	$ 11,505
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total share-based compensation expense	5,253	6,616	4,009
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total share-based compensation expense	$ 3,470	$ 4,800	6,447
Restructuring Charges [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total share-based compensation expense			$ 1,049

401(k) Savings Plan - Additiona

401(k) Savings Plan - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Compensation And Retirement Disclosure [Abstract]
Contribution by company	$ 0.3	$ 0.3	$ 0.1

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Taxes [Line Items]
Income tax benefit of expense		$ 0	$ 775,000	$ 0
Statutory federal income tax rate		34.00%
Provisional tax expense		$ 0
Deferred tax balance		$ 10,200,000
One-time transition tax rate on cash assets		15.50%
One-time transition tax rate for other specified assets		8.00%
One-time transition tax liability for foreign subsidiaries		$ 0
Increase (decrease) in valuation allowance due to net operating loss		12,000,000	(5,300,000)
Unrecognized tax benefits		2,745,000	2,157,000	1,568,000	$ 471,000
Unrecognized tax benefits, if recognized would effective annual tax rate		0
Accrued interest or penalties related to uncertain tax positions		$ 0	0
Interest or penalties recognized			$ 0	$ 0
Minimum [Member]
Income Taxes [Line Items]
Tax examinations, year under examination		2,012
Maximum [Member]
Income Taxes [Line Items]
Tax examinations, year under examination		2,017
Domestic Tax Authority [Member]
Income Taxes [Line Items]
NOL carryforwards		$ 53,200,000
NOL carryforwards expiration		expire at various years beginning with 2036
Domestic Tax Authority [Member] | Research Tax Credit Carryforward [Member]
Income Taxes [Line Items]
Tax credit carryforwards		$ 800,000
Tax credit carryforwards expiration		expire at various years beginning with 2036
State credit carryforwards [Member]
Income Taxes [Line Items]
NOL carryforwards		$ 37,800,000
NOL carryforwards expiration		expire at various years beginning with 2036
State credit carryforwards [Member] | Research Tax Credit Carryforward [Member]
Income Taxes [Line Items]
Tax credit carryforwards		$ 1,000,000
Foreign Tax Authority [Member]
Income Taxes [Line Items]
NOL carryforwards		$ 44,100,000
Scenario Forecast [Member]
Income Taxes [Line Items]
Statutory federal income tax rate	21.00%

Income Taxes - Schedule of Comp

Income Taxes - Schedule of Components of Loss Before Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
U.S.	$ (36,923)	$ (96,498)	$ (47,235)
Foreign	(19,224)	(18,024)	(218)
Net loss before income taxes	$ (56,147)	$ (114,522)	$ (47,453)

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of Income Tax Expense Computed Statutory Federal Income Tax and Financial Statements (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal income tax expense at statutory rate	$ (19,090)	$ (38,938)	$ (16,134)
Non-deductible foreign research expenses	19	21	45
Stock-based compensation	(76)	1,356	1,418
Non-deductible expenses	61	985	295
Goodwill impairment		16,834
Research and development tax credits	(507)	(326)	(910)
Change in valuation allowance	7,935	16,062	15,277
Foreign rate differential	1,495	3,231	9
Rate change	10,163
Total tax benefit	$ 0	$ (775)	$ 0

Income Taxes - Schedule of Defe

Income Taxes - Schedule of Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Net operating loss carryforwards	$ 27,142	$ 15,785	$ 19,889
Accruals, reserve and other	2,586	3,071	2,681
Stock-based compensation	3,623	3,848	4,328
Tax credit carryforwards	1,774	574	1,945
Property and equipment	274	54
Intangibles	64	68	45
Total deferred tax assets before valuation allowance	35,463	23,400	28,888
Valuation allowance	(35,463)	(23,400)	(28,839)
Total deferred tax assets			49
Deferred tax liabilities:
Property and equipment			(49)
IPR&D	(1,250)	(1,250)
Total deferred tax liabilities	$ (1,250)	(1,250)	(49)
Net deferred tax assets		$ 0	$ 0

Income Taxes - Schedule of Unre

Income Taxes - Schedule of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Unrecognized tax benefits as of the beginning of the year	$ 2,157	$ 1,568	$ 471
Increase (decrease) related to prior year tax provisions			172
Increase related to current year tax provisions	588	589	925
Unrecognized tax benefits as of the end of the year	$ 2,745	$ 2,157	$ 1,568

Net Loss per Share - Schedule o

Net Loss per Share - Schedule of Antidilutive Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive common stock equivalents excluded from calculation of diluted net loss per share	9,371	8,595	6,208
Stock Options [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive common stock equivalents excluded from calculation of diluted net loss per share	6,695	7,449	5,494
Restricted Stock Units [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive common stock equivalents excluded from calculation of diluted net loss per share	2,515	1,049	632
ESPP [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive common stock equivalents excluded from calculation of diluted net loss per share	71	47	42
Warrants to Purchase Common Stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive common stock equivalents excluded from calculation of diluted net loss per share	90	50	40

Subsequent Events- Additional I

Subsequent Events- Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended			12 Months Ended
	Feb. 28, 2018	Jan. 31, 2018	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2015	Jan. 25, 2018	Aug. 31, 2016
Subsequent Event [Line Items]
Issuance of common stock			230,000
Proceeds from sales of common stock			$ 8,300	$ 16,519	$ 138,954
2017 Stock Offering Agreement [Member]
Subsequent Event [Line Items]
Issuance of common stock				5,130,339
Proceeds from sales of common stock				$ 16,500
Common Stock [Member]
Subsequent Event [Line Items]
Issuance of common stock				5,130,339	2,599,375
Editas Medicine Inc [Member]
Subsequent Event [Line Items]
Collaboration agreement extension description				On January 25, 2018, the Company and Editas extended the research collaboration, option and license agreement. In consideration for extending the agreement, Editas made a one-time payment, non-refundable cash payment of $0.5 million to the Company in February 2018.
Collaboration, option and license agreement fee receivable per indication							$ 1,000
Subsequent Event [Member] | 2017 Stock Offering Agreement [Member]
Subsequent Event [Line Items]
Issuance of common stock		1,419,893
Proceeds from sales of common stock		$ 5,700
Subsequent Event [Member] | Common Stock [Member] | 2017 Stock Offering Agreement [Member]
Subsequent Event [Line Items]
Issuance of common stock	10,222,235	1,419,893
Proceeds from sales of common stock	$ 64,300	$ 5,700
Subsequent Event [Member] | Editas Medicine Inc [Member]
Subsequent Event [Line Items]
Proceeds from non-refundable cash payment for extending agreement	$ 500	$ 500
Collaboration, option and license agreement designated initial indication fees receivable						$ 1,300
Collaboration, option and license agreement additional indication fees receivable						1,500
Collaboration, option and license agreement fee receivable per indication						$ 1,000

Selected Quarterly Financial 78

Selected Quarterly Financial Information - Schedule of Quarterly Consolidated Results of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 461	$ 463	$ 463	$ 462	$ 488	$ 395	$ 307	$ 265	$ 1,849	$ 1,455	$ 2,319
Total operating expenses	(16,056)	(15,034)	(12,556)	(17,050)	(23,875)	(14,902)	(62,189)	(15,773)	(60,696)	(116,739)	(50,142)
Net loss	$ (14,789)	$ (13,829)	$ (11,430)	$ (16,099)	$ (22,394)	$ (14,301)	$ (61,660)	$ (15,392)	$ (56,147)	$ (113,747)	$ (47,453)
Basic and diluted net loss per share	$ (0.32)	$ (0.32)	$ (0.27)	$ (0.38)	$ (0.54)	$ (0.35)	$ (1.76)	$ (0.57)	$ (1.29)	$ (3.14)	$ (1.86)

Selected Quarterly Financial 79

Selected Quarterly Financial Information - Schedule of Quarterly Consolidated Results of Operations (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended						12 Months Ended
	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Quarterly Financial Information [Line Items]
Goodwill impairment charge				$ 400	$ 49,100
Impairment charge			$ 11,200
Stock-based compensation charge						$ 2,200	$ 8,723	$ 11,416	$ 11,505
Cornell University [Member] | Master Service Agreement [Member]
Schedule Of Quarterly Financial Information [Line Items]
Estimated costs associated with the termination		$ 2,300					$ 2,000
Estimated costs adjustments associated with the termination	$ 300