Regulus Therapeutics (RGLS) 10-K 3 Mar 17 2016 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Feb. 24, 2017	Jun. 30, 2016
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	FY
Trading Symbol	RGLS
Entity Registrant Name	REGULUS THERAPEUTICS INC.
Entity Central Index Key	1,505,512
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 152.6
Entity Common Stock, Shares Outstanding		52,932,405

Balance Sheets

Balance Sheets - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 14,941,000	$ 15,960,000
Short-term investments	61,170,000	98,103,000
Restricted cash	0	1,256,000
Contract and other receivables	1,657,000	10,021,000
Prepaid materials, net	5,552,000	5,543,000
Prepaid expenses and other current assets	4,154,000	3,375,000
Total current assets	87,474,000	134,258,000
Property and equipment, net	11,830,000	5,400,000
Intangibles, net	1,015,000	1,081,000
Other assets	342,000	344,000
Total assets	100,661,000	141,083,000
Current liabilities:
Accounts payable	5,840,000	2,717,000
Accrued liabilities	5,577,000	6,329,000
Accrued compensation	2,318,000	2,392,000
Current portion of deferred revenue	72,000	1,194,000
Total current liabilities	13,807,000	12,632,000
Term loan, less debt issuance costs	19,802,000	0
Deferred revenue, less current portion	1,993,000	2,065,000
Deferred rent, less current portion	8,840,000	2,308,000
Other long-term liabilities	144,000	0
Total liabilities	44,586,000	17,005,000
Commitments and Contingencies (Note 8)
Stockholders’ equity:
Common stock, $0.001 par value; 200,000,000 shares authorized, 52,924,805 and 52,669,266 shares issued and outstanding at December 31, 2016 and 2015, respectively	53,000	53,000
Additional paid-in capital	329,496,000	315,673,000
Accumulated other comprehensive loss	(123,000)	(133,000)
Accumulated deficit	(273,351,000)	(191,515,000)
Total stockholders’ equity	56,075,000	124,078,000
Total liabilities and stockholders’ equity	$ 100,661,000	$ 141,083,000

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	52,924,805	52,669,266
Common stock, shares outstanding	52,924,805	52,669,266

Statements of Operations and Co

Statements of Operations and Comprehensive Loss - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues:
Total revenues	$ 1,194,000	$ 20,759,000	$ 7,669,000
Operating expenses:
Research and development	64,305,000	56,387,000	41,046,000
General and administrative	18,391,000	19,130,000	11,533,000
Total operating expenses	82,696,000	75,517,000	52,579,000
Loss from operations	(81,502,000)	(54,758,000)	(44,910,000)
Other income (expense):
Interest and other income	844,000	855,000	388,000
Interest and other expense	(1,182,000)	(52,000)	(39,000)
Loss from valuation of convertible note payable	0	(1,811,000)	(12,118,000)
Loss before income taxes	(81,840,000)	(55,766,000)	(56,679,000)
Income tax benefit (expense)	4,000	18,000	(1,000)
Net loss	(81,836,000)	(55,748,000)	(56,680,000)
Other comprehensive loss:
Unrealized gain (loss) on short-term investments, net	10,000	64,000	(181,000)
Comprehensive loss	$ (81,826,000)	$ (55,684,000)	$ (56,861,000)
Net loss per share - basic and diluted (in dollars per share)	$ (1.55)	$ (1.08)	$ (1.29)
Weighted average shares used to compute basic and diluted net loss per share (in shares)	52,813,474	51,411,353	44,090,165
Strategic Alliances and Collaborations
Revenues:
Total revenues	$ 1,194,000	$ 20,759,000	$ 7,669,000

Statements of Stockholders' Equ

Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Private Placement	Common stock	Common stockPrivate Placement	Additional paid-in capital	Additional paid-in capitalPrivate Placement	Accumulated other comprehensive income (loss)	Accumulated deficit
Beginning balance at Dec. 31, 2013	$ 93,457		$ 42		$ 172,518		$ (16)	$ (79,087)
Beginning balance (in shares) at Dec. 31, 2013			41,787,326
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of options (in shares)			1,006,515
Issuance of common stock upon exercise of options	2,233		$ 1		2,232
Stock-based compensation expense	7,039				7,039
Issuance of common stock under Employee Stock Purchase Plan (in shares)			38,085
Issuance of common stock under Employee Stock Purchase Plan	283				283
Issuance of common stock (in shares)			4,808,824	1,303,780
Issuance of common stock	76,294	$ 9,569	$ 5	$ 1	76,289	$ 9,568
Unrealized gain (loss) on short-term investments	(181)						(181)
Net loss	(56,680)							(56,680)
Ending balance at Dec. 31, 2014	132,014		$ 49		267,929		(197)	(135,767)
Ending balance (in shares) at Dec. 31, 2014			48,944,530
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of options (in shares)			2,298,618
Issuance of common stock upon exercise of options	6,680		$ 2		6,678
Stock-based compensation expense	15,368				15,368
Issuance of common stock under Employee Stock Purchase Plan (in shares)			69,380
Issuance of common stock under Employee Stock Purchase Plan	492				492
Conversion of convertible note payable into common stock (in shares)			1,356,738
Conversion of convertible note payable into common stock	25,208		$ 2		25,206
Unrealized gain (loss) on short-term investments	64						64
Net loss	(55,748)							(55,748)
Ending balance at Dec. 31, 2015	$ 124,078		$ 53		315,673		(133)	(191,515)
Ending balance (in shares) at Dec. 31, 2015			52,669,266
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of options (in shares)	90,000		89,855
Issuance of common stock upon exercise of options	$ 310				310
Stock-based compensation expense	12,872				12,872
Issuance of common stock under Employee Stock Purchase Plan (in shares)			165,684
Issuance of common stock under Employee Stock Purchase Plan	641				641
Unrealized gain (loss) on short-term investments	10						10
Net loss	(81,836)							(81,836)
Ending balance at Dec. 31, 2016	$ 56,075		$ 53		$ 329,496		$ (123)	$ (273,351)
Ending balance (in shares) at Dec. 31, 2016			52,924,805

Statements of Stockholders' Eq6

Statements of Stockholders' Equity (Parentheticals) $ in Thousands	12 Months Ended
	Dec. 31, 2014USD ($)
Statement of Stockholders' Equity [Abstract]
Issuance of common stock, offering costs	$ 347

Statements of Cash Flows

Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities
Net loss	$ (81,836,000)	$ (55,748,000)	$ (56,680,000)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization expense	2,276,000	1,591,000	1,490,000
Loss from valuation of convertible note payable	0	1,811,000	12,118,000
Stock-based compensation	12,872,000	15,368,000	7,039,000
Amortization of premium on investments, net	666,000	1,472,000	1,597,000
Other	1,043,000	98,000	18,000
Change in operating assets and liabilities:
Contracts and other receivables	8,364,000	(9,746,000)	(196,000)
Prepaid materials	(616,000)	(3,460,000)	(641,000)
Prepaid expenses and other assets	(778,000)	(683,000)	(1,241,000)
Accounts payable	3,123,000	529,000	940,000
Accrued liabilities	(871,000)	2,094,000	556,000
Accrued compensation	(74,000)	284,000	811,000
Deferred revenue	(1,194,000)	(3,090,000)	(5,039,000)
Deferred rent and other liabilities	143,000	(379,000)	(282,000)
Net cash used in operating activities	(56,882,000)	(49,859,000)	(39,510,000)
Investing activities
Purchases of short-term investments	(65,110,000)	(78,401,000)	(113,417,000)
Sales and maturities of short-term investments	101,387,000	101,306,000	85,421,000
Purchases of property and equipment	(913,000)	(1,363,000)	(1,146,000)
Acquisition of intangibles	(53,000)	(67,000)	(65,000)
Net cash provided by (used in) investing activities	35,311,000	21,475,000	(29,207,000)
Financing activities
Proceeds from borrowing under term loan, net	19,768,000	0	0
Proceeds from issuance of common stock, net	641,000	492,000	86,146,000
Proceeds from exercise of common stock options	310,000	6,680,000	2,233,000
Principal payments on other long-term obligations	(167,000)	(155,000)	(142,000)
Net cash provided by financing activities	20,552,000	7,017,000	88,237,000
Net (decrease) increase in cash and cash equivalents	(1,019,000)	(21,367,000)	19,520,000
Cash and cash equivalents at beginning of period	15,960,000	37,327,000	17,807,000
Cash and cash equivalents at end of period	14,941,000	15,960,000	37,327,000
Supplemental disclosure of cash flow information
Net changes in restricted cash	(1,256,000)	1,256,000	0
Interest paid	(981,000)	(27,000)	(39,000)
Income taxes paid	(1,000)	(1,000)	(1,000)
Supplemental disclosure of non-cash investing and financing activities
Allowance for tenant improvements	6,653,000	1,656,000	0
Amounts accrued for property and equipment	292,000	223,000	76,000
Amounts accrued for patent expenditures	$ 0	$ 28,000	$ 42,000

The Business, Basis of Presenta

The Business, Basis of Presentation and Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
The Business, Basis of Presentation and Summary of Significant Accounting Policies	The Business, Basis of Presentation and Summary of Significant Accounting Policies We are a biopharmaceutical company focused on discovering and developing first-in-class drugs that target microRNAs to treat a broad range of diseases. We were formed in 2007 when Alnylam Pharmaceuticals, Inc. and Ionis Pharmaceuticals, Inc., contributed significant intellectual property, know-how and financial and human capital to pursue the development of drugs targeting micro RNAs pursuant to a license and collaboration agreement. Regulus Therapeutics Inc. was converted to a Delaware corporation on January 2, 2009. As used in this report, unless the context suggests otherwise, “the Company,” “our,” “us” and “we” means Regulus Therapeutics Inc. Use of Estimates Our financial statements are prepared in accordance with United States Generally Accepted Accounting Principles, which requires us to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses and the disclosure of contingent assets and liabilities in our financial statements and accompanying notes. An estimated loss contingency is accrued in our financial statements if it is probable that a liability has been incurred and the amount of the loss can be reasonably estimated. Although these estimates are based on our knowledge of current events and actions we may undertake in the future, actual results may ultimately differ from these estimates and assumptions. Revenue Recognition Our revenues generally consist of upfront payments for licenses or options to obtain licenses in the future, milestone payments and payments for other research services under strategic alliance and collaboration agreements. We recognize revenues when all four of the following criteria are met: (1) persuasive evidence of an arrangement exists; (2) delivery of the products and/or services has occurred; (3) the selling price is fixed or determinable; and (4) collectability is reasonably assured. Multiple element arrangements, such as our strategic alliance agreements with Sanofi and AstraZeneca AB (“AstraZeneca”), are analyzed to determine whether the deliverables within the agreement can be separated or whether they must be accounted for as a single unit of accounting. Deliverables under the agreement will be accounted for as separate units of accounting provided that (i) a delivered item has value to the customer on a stand-alone basis; and (ii) if the agreement includes a general right of return relative to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in the control of the vendor. The allocation of consideration amongst the deliverables under the agreement is derived using a “best estimate of selling price” if vendor specific objective evidence and third-party evidence of fair value is not available. If the delivered element does not have stand-alone value, the arrangement is then accounted for as a single unit of accounting, and we recognize the consideration received under the arrangement as revenue on a straight-line basis, which approximates effort over our estimated period of performance, which for us is typically the expected term of the research and development plan. Milestones We apply the milestone method of accounting to recognize revenue from milestone payments when earned, as evidenced by written acknowledgement from the collaborator or other persuasive evidence that the milestone has been achieved and the payment is non-refundable, provided that the milestone event is substantive. A milestone event is defined as an event (i) that can only be achieved based in whole or in part on either our performance or on the occurrence of a specific outcome resulting from our performance; (ii) for which there is substantive uncertainty at the inception of the arrangement that the event will be achieved; and (iii) that would result in additional payments being due to us. Events for which the occurrence is either contingent solely upon the passage of time or the result of a counterparty’s performance are not considered to be milestone events. A milestone event is substantive if all of the following conditions are met: (i) the consideration is commensurate with either our performance to achieve the milestone, or the enhancement of the value to the delivered item(s) as a result of a specific outcome resulting from our performance to achieve the milestone; (ii) the consideration relates solely to past performance; and (iii) the consideration is reasonable relative to all the deliverables and payment terms (including other potential milestone consideration) within the arrangement. We assess whether a milestone is substantive at the inception of each arrangement. If a milestone is deemed non-substantive, we will account for that milestone payment using a method consistent with the related units of accounting for the arrangement over the estimated performance period. Deferred Revenue Amounts received prior to satisfying the above revenue recognition criteria are recorded as deferred revenue in the accompanying balance sheets. Amounts not expected to be recognized within the next 12 months are classified as non-current deferred revenue. Stock-Based Compensation We account for stock-based compensation expense related to stock options granted to employees and members of our board of directors by estimating the fair value of each stock option on the date of grant using the Black-Scholes option pricing model. We recognize stock-based compensation expense using the accelerated multiple-option approach. Under the accelerated multiple-option approach (also known as the graded-vesting method), we recognize compensation expense over the requisite service period for each separately vesting tranche of the award as though the award was in substance multiple awards, resulting in accelerated expense recognition over the vesting period. For performance-based awards granted to employees (i) the fair value of the award is determined on the grant date, (ii) we assess the probability of the individual milestones under the award being achieved and (iii) the fair value of the shares subject to the milestone is expensed over the implicit service period commencing once management believes the performance criteria is probable of being met. We account for stock options granted to non-employees using the fair value approach. Stock options granted to non-employees are subject to periodic revaluation over their vesting terms. Prepaid Materials We capitalize the purchase of certain raw materials and related supplies for use in the manufacturing of drug product in our clinical development programs, as we have determined that these materials have alternative future use. We can use these raw materials and related supplies in multiple clinical drug products, and therefore have future use independent of the development status of any particular drug program until it is utilized in the manufacturing process. We periodically review these capitalized materials for indicators of impairment, including shelf life, continued alternative future use and obsolescence. As of December 31, 2016 and 2015 , our prepaid materials balance was $5.6 million , net of a $0.6 million reserve, and $5.5 million , respectively. Research and Development Research and development costs are expensed as incurred and consist of costs associated with research activities supporting our drug discovery efforts, compensation and related benefits, non-cash stock-based compensation, license fees, laboratory supplies and associated overhead and facility costs. Income Taxes Income taxes are accounted for under the asset and liability method. This approach requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of the differences between the tax basis of assets or liabilities and their carrying amounts in the financial statements using the enacted tax rates and laws that are anticipated to be in effect when the differences are expected to reverse. We provide a valuation allowance against net deferred tax assets if it is more likely than not that these items will either expire before the Company is able to realize their benefit or if future deductibility is uncertain. In accordance with the accounting standards for uncertain tax positions, the Company evaluates the recognition threshold and measurement attribute criteria for the financial statement recognition and measurement of tax positions taken or expected to be taken in a tax return. For those benefits to be recognized, a tax position must be more likely than not to be sustained upon examination by taxing authorities. Fair Value Option Applicable accounting policies permit entities to choose, at specified election dates, to measure specified items at fair value if the decision about the election is: (1) applied instrument by instrument, (2) irrevocable, and (3) applied to an entire instrument. The balance of our convertible note payable, which was valued under the fair value option, was converted into shares of common stock in January 2015 (see Note 9). Clinical Trial and Preclinical Study Accruals We make estimates of our accrued expenses for clinical trial and preclinical study activities as of each balance sheet date in our financial statements based on the facts and circumstances known to us at that time. These accruals are based upon estimates of costs incurred and fees that may be associated with services provided by clinical trial investigational sites, clinical research organizations (“CROs”) and for other clinical trial-related activities. Payments under certain contracts with such parties depend on factors such as successful enrollment of patients, site initiation and the completion of clinical trial milestones. In accruing for these services, we estimate the time period over which services will be performed and the level of effort to be expended in each period. If possible, we obtain information regarding unbilled services directly from these service providers. However, we may be required to estimate these services based on other information available to us. If we underestimate or overestimate the activities or fees associated with a study or service at a given point in time, adjustments to research and development expenses may be necessary in future periods. Historically, our estimated accrued liabilities have approximated actual expense incurred. Subsequent changes in estimates may result in a material change in our accruals. Cash and Cash Equivalents We classify time deposits and other investments that are highly liquid and have maturities of 90 days or less at the date of purchase as cash equivalents. The carrying amounts approximate fair value due to the short maturities of these instruments. Restricted Cash Restricted cash consists of amounts received for a specific and limited purpose, and therefore not available for general operating activities. In August 2015, we received $1.4 million in connection with our facility lease agreement with Walton Torrey Owner B, L.L.C, entered into in July 2015. The use of these funds were restricted to costs associated with the relocation of our corporate headquarters. As of December 31, 2016, our restricted cash balance was zero . Short-Term Investments We carry short-term investments classified as available-for-sale at fair value as determined by prices for identical or similar securities at the balance sheet date. Our short-term investments consist of both Level 1 and Level 2 financial instruments in the fair value hierarchy. We record unrealized gains and losses as a component of other comprehensive loss within the statements of operations and comprehensive loss and as a separate component of stockholders’ equity. We determine the realized gains or losses of available-for-sale securities using the specific identification method and include net realized gains and losses in interest income. At each balance sheet date, we assess available-for-sale securities in an unrealized loss position to determine whether the unrealized loss is other-than-temporary. We consider factors including: the significance of the decline in value compared to the cost basis, underlying factors contributing to a decline in the prices of securities in a single asset class, the length of time the market value of the security has been less than its cost basis, the security’s relative performance versus its peers, sector or asset class, expected market volatility and the market and economy in general. When we determine that a decline in the fair value below its cost basis is other-than-temporary, we recognize an impairment loss in the year in which the other-than-temporary decline occurred. We determined that there were no other-than-temporary declines in the value of short-term investments as of December 31, 2016 or 2015 . Concentrations of Credit Risk Financial instruments that potentially subject us to significant concentrations of credit risk consist primarily of cash equivalents and short-term investments. We maintain deposits in federally insured financial institutions in excess of federally insured limits. We have not experienced any material losses in such accounts and believe we are not exposed to significant risk. We maintain our cash equivalents and short-term investments with two highly accredited financial institutions. We invest our excess cash primarily in commercial paper, certificates of deposit and debt instruments of financial institutions and corporations, United States Treasury securities and United States government-sponsored enterprise securities. Additionally, we adhere to established guidelines regarding approved investments and maturities of investments, which are designed to preserve their principal value and maintain liquidity. Property and Equipment We carry our property and equipment at cost, which consists of lab equipment, computer equipment and software, furniture and fixtures and leasehold improvements. Property and equipment is depreciated using the straight-line method over the estimated useful lives (generally three to five years). Leasehold improvements are amortized over the lesser of their useful life or the remaining lease term, including any renewal periods that are deemed to be reasonably assured. Repair and maintenance costs that do not improve service potential or extend economic life are expensed as incurred. Intangibles We capitalize costs which consist principally of outside legal costs and filing fees related to obtaining patents. We review our capitalized patent costs periodically to determine that they include costs for patent applications that have future value and an alternative future use. We evaluate costs related to patents that we are not actively pursuing and write off these costs. We amortize patent costs over their patent lives, beginning with the date the patents are issued. The weighted average remaining life of the issued patents was approximately 10 years at December 31, 2016 . We obtain licenses from third parties and capitalize the costs related to exclusive licenses that have alternative future use within multiple potential programs. We amortize capitalized licenses over their estimated useful life or term of the agreement, which for current licenses is 10 years. Impairment of Long-Lived Assets We regularly review the carrying amount of our property, equipment and intangible assets to determine whether indicators of impairment may exist which warrant adjustments to carrying values or estimated useful lives. If indications of impairment exist, projected future undiscounted cash flows associated with the asset are compared to the carrying amount to determine whether the asset’s value is recoverable. If the carrying value of the asset exceeds such projected undiscounted cash flows, the asset will be written down to its estimated fair value. No impairment charges were recorded during the years ended December 31, 2016 , 2015 or 2014 . Segment Reporting Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker in making decisions regarding resource allocation and assessing performance. To date, we have viewed our operations and managed our business as one segment operating primarily within the United States. Comprehensive Loss Comprehensive loss is defined as the change in equity during a period from transactions and other events and/or circumstances from non-owner sources. Our only component of other comprehensive loss is unrealized gains (losses) on available-for-sale securities. Comprehensive gains (losses) have been reflected in the statements of operations and comprehensive loss and as a separate component in the statements of stockholders’ equity for all periods presented. Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2014-09, Revenue from Contracts with Customers (Topic 606) , which outlines a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and supersedes most current revenue recognition guidance, including industry-specific guidance. ASU 2014-09 outlines a five-step process for revenue recognition that focuses on transfer of control, as opposed to transfer of risk and rewards, and also requires enhanced disclosures regarding the nature, amount, timing and uncertainty of revenues and cash flows from contracts with customers. Major provisions include determining which goods and services are distinct and require separate accounting (performance obligations), how variable consideration (which may include change orders and claims) is recognized, whether revenue should be recognized at a point in time or over time and ensuring the time value of money is considered in the transaction price. The FASB issued supplemental adoption guidance and clarification to ASU 2014-09 in March 2016, April 2016 and May 2016 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 and ASU 2016-12, Revenue from Contracts with Customers: Narrow-Scope Improvements and Practical Expedients , respectively. ASU 2014-09 and the related supplemental ASUs are effective for fiscal years beginning after December 15, 2017 and interim periods therein. The ASU permits two methods of adoption: the full retrospective method or the modified retrospective method. We currently plan on applying the modified retrospective method upon adoption in the first quarter of 2018, and currently do not anticipate that the adoption of this ASU will have a material impact with regard to our current contracts. In August 2014, the FASB issued ASU No. 2014-15, Presentation of Financial Statements Going Concern , which requires management to assess an entity’s ability to continue as a going concern, and to provide related footnote disclosure in certain circumstances. This standard is effective for annual reporting periods ending after December 15, 2016 and interim periods thereafter. The adoption of this guidance had no impact on our financial statements. In April 2015, the FASB issued ASU No. 2015-03, Interest- Imputation of Interest: Simplifying the Presentation of Debt Issuance Costs, which requires that debt issuance costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from the carrying amount of that debt liability, consistent with debt discounts. The recognition and measurement guidance for debt issuance costs were not affected by the amendments in ASU No. 2015-03. In June 2016, upon entering into a loan and security agreement, we adopted ASU No. 2015-03, which resulted in the classification of $0.2 million of debt issuance costs against the principal balance of our outstanding term loan of $20.0 million . In January 2016, the FASB issued ASU No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities , which eliminates the requirement for public companies to disclose the method(s) and significant assumptions used to estimate the fair value that is required to be disclosed for financial instruments measured at amortized cost on the balance sheet. Additionally, the standard requires public companies to use the exit price notion when measuring the fair value of financial instruments for disclosure purposes. Furthermore, the standard requires presentation of financial assets and liabilities by measurement category and form of financial asset on the balance sheet or accompanying notes to the financial statements. The standard is effective for annual reporting periods beginning after December 15, 2017, including interim periods within those annual reporting periods. Early application is permitted. The adoption of this guidance will have no impact on our financial statements. In February 2016, the FASB issued ASU No. 2016-02, Leases, which increases transparency and comparability among organizations by requiring recognition of lease assets and lease liabilities on the balance sheet and disclosure of key information about leasing arrangements. The standard is effective for annual reporting periods beginning after December 15, 2018, including interim periods within those annual reporting periods. Early application is permitted. We are currently evaluating the impact of adoption on our financial statements. In March 2016, the FASB issued ASU No. 2016-09, Compensation - Stock Compensation: Improvements to Employee Share-Based Payment Accounting, which is intended to simplify several aspects of accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. The standard is effective for annual reporting periods beginning after December 15, 2016, and interim periods within those annual reporting periods. Early application is permitted. We intend to adopt this guidance in the first quarter of 2017. We expect that the adoption of this guidance will increase our deferred tax assets with a corresponding increase to our valuation allowance. We maintained a full valuation allowance against our deferred tax assets at December 31, 2016. In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments , which addresses the presentation and classification of certain cash receipts and cash payments in the statement of cash flows under Accounting Standards Codification 230. The standard is effective for annual reporting periods beginning after December 15, 2017, and interim periods within those annual reporting periods. Early application is permitted. The adoption of this guidance will have no impact on our financial statements. In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows: Restricted Cash , which requires restricted cash and restricted cash equivalents to be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. The standard is effective for annual reporting periods beginning after December 15, 2017, and interim periods within those annual reporting periods. Early application is permitted. We have not elected to early adopt this standard as of December 31, 2016. Upon adoption, the 2016 period in our three-year statements of cash flows will include $1.3 million of restricted cash in cash and cash equivalents at the beginning of the period. We do not expect any additional impact on our financial statements.

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2016
Earnings Per Share [Abstract]
Net Loss Per Share	Net Loss Per Share Basic net loss per share is calculated by dividing the net loss by the weighted average number of common shares outstanding for the period, without consideration for common stock equivalents. Diluted net loss per share is calculated by dividing the net loss by the weighted-average number of common share equivalents outstanding for the period determined using the treasury-stock method. Dilutive common stock equivalents are comprised of options outstanding under our stock option plans and convertible note payable, which was converted into common stock in January 2015. For all periods presented, there is no difference in the number of shares used to calculate basic and diluted net loss per share. Potentially dilutive securities not included in the calculation of diluted net loss per share because to do so would be anti-dilutive are as follows (in common equivalent shares): Years ended December 31, 2016 2015 2014 Common stock options 5,553,210 1,674,119 2,566,423 Convertible note payable — — 1,461,474 Total 5,553,210 1,674,119 4,027,897

Investments

Investments	12 Months Ended
	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Investments	Investments We invest our excess cash in commercial paper and debt instruments of financial institutions and corporations, United States Treasury securities and United States government-sponsored enterprise securities. As of December 31, 2016 , our short-term investments had a weighted average maturity of less than two years. The following tables summarize our short-term investments (in thousands): Maturity (in years) Amortized cost Unrealized Estimated fair value Gains Losses As of December 31, 2016 Corporate debt securities 2 or less $ 49,185 $ 12 $ (77 ) $ 49,120 Certificates of deposit 1 or less 9,291 — — 9,291 Commercial paper 1 or less 1,247 — — 1,247 U.S. treasury securities 1 or less 1,001 — (1 ) 1,000 Debt securities of U.S. government-sponsored agencies 1 or less 512 — — 512 Total $ 61,236 $ 12 $ (78 ) $ 61,170 Maturity (in years) Amortized cost Unrealized Estimated fair value Gains Losses As of December 31, 2015 Corporate debt securities 2 or less $ 81,054 $ 16 $ (103 ) $ 80,967 Certificates of deposit 2 or less 13,640 — — 13,640 Commercial paper 1 or less 3,490 6 — 3,496 Total $ 98,184 $ 22 $ (103 ) $ 98,103

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements We have certain financial assets and liabilities recorded at fair value which have been classified as Level 1, 2, or 3 within the fair value hierarchy as described in the accounting standards for fair value measurements. Accounting standards define fair value as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants as of the measurement date. Market participants are buyers and sellers in the principal market that are (i) independent, (ii) knowledgeable, (iii) able to transact, and (iv) willing to transact. The accounting standards provide an established hierarchy for inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the most observable inputs be used when available. Observable inputs are inputs that market participants would use in valuing the asset or liability and are developed based on market data obtained from independent sources. Unobservable inputs are inputs that reflect our assumptions about the factors that market participants would use in valuing the asset or liability. The accounting standards prioritize the inputs used in measuring the fair value into the following hierarchy: • Level 1 includes financial instruments for which quoted market prices for identical instruments are available in active markets. • Level 2 includes financial instruments for which there are inputs other than quoted prices included within Level 1 that are observable for the instrument such as quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets with insufficient volume or infrequent transactions (less active markets) or model-driven valuations in which significant inputs are observable or can be derived principally from, or corroborated by, observable market data. • Level 3 includes financial instruments for which fair value is derived from valuation techniques in which one or more significant inputs are unobservable, including management’s own assumptions. The following table presents our fair value hierarchy for assets and liabilities measured at fair value on a recurring basis as of December 31, 2016 and December 31, 2015 (in thousands): Fair value as of December 31, 2016 Total Level 1 Level 2 Level 3 Assets: Cash equivalents $ 13,578 $ 13,578 $ — $ — Corporate debt securities 49,120 — 49,120 — Certificates of deposit 9,291 — 9,291 — Commercial paper 1,247 — 1,247 — U.S. treasury securities 1,000 — 1,000 — Debt securities of U.S. government-sponsored agencies 512 — 512 — $ 74,748 $ 13,578 $ 61,170 $ — Fair value as of December 31, 2015 Total Level 1 Level 2 Level 3 Assets: Cash equivalents $ 15,152 $ 15,152 $ — $ — Corporate debt securities 80,967 — 80,967 — Certificates of deposit 13,640 — 13,640 — Commercial paper 3,496 — 3,496 — $ 113,255 $ 15,152 $ 98,103 $ — We obtain pricing information from quoted market prices or quotes from brokers/dealers. We generally determine the fair value of our investment securities using standard observable inputs, including reported trades, broker/dealer quotes, bids and/or offers. Refer to Note 3 for information regarding our investments.

Strategic Alliances and Collabo

Strategic Alliances and Collaborations	12 Months Ended
	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Strategic Alliances and Collaborations	Strategic Alliances and Collaborations The following table summarizes our total revenues from our strategic alliances and collaborations during the periods presented (in thousands): Year ended December 31, 2016 2015 2014 AstraZeneca $ 1,122 $ 18,871 $ 1,859 Sanofi 72 71 979 GSK — — 3,509 Biogen — 1,817 1,122 Other — — 200 Total $ 1,194 $ 20,759 $ 7,669 AstraZeneca In August 2012, we entered into a collaboration and license agreement with AstraZeneca. Under the terms of the agreement, we agreed to collaborate with AstraZeneca to identify, research and develop compounds targeting three micro RNA alliance targets primarily in the fields of cardiovascular diseases, metabolic diseases and oncology. Pursuant to the agreement, we granted AstraZeneca an exclusive, worldwide license to develop, manufacture and commercialize lead compounds designated by AstraZeneca in the course of the collaboration activities against the alliance targets for all human therapeutic uses. Under the terms of the agreement we were required to use commercially reasonable efforts to perform all research, development and manufacturing activities described in the research plan, at our cost, until the acceptance of an investigational new drug application ("IND") or the end of the research term, which expired in August 2016. Under the terms of the agreement, we received an upfront payment of $3.0 million in October 2012. We determined the elements within the agreement should be treated as a single unit of accounting because the delivered element, the license, did not have stand-alone value. As a result, we recognized revenue related to the upfront payment on a straight-line basis over the period of performance, which was four years based on the term of the research and development plan which ended in August 2016. In connection with the collaboration and license agreement and concurrently with our initial public offering, we sold AstraZeneca 6,250,000 shares of our common stock in a private placement at a price per share of $4.00 . Under the terms of the Common Stock Purchase Agreement ("CSPA"), AstraZeneca could not sell, transfer, make any short sale of, or grant any option for the sale of any common stock for a 365 -day period following the effective date of our initial public offering. The CSPA and collaboration and license agreement were negotiated concurrently and were therefore evaluated as a single agreement. Based upon restricted stock studies of similar duration and a Black-Scholes valuation to measure a discount for lack of marketability, $4.3 million was attributed to the collaboration and license agreement. We recognized the $4.3 million into revenue ratably over the period of performance of the collaboration of four years, which ended in August 2016. In March 2015, we earned a $2.5 million preclinical milestone payment upon AstraZeneca’s selection of RG-125(AZD4076), a GalNAc-conjugated anti-miR targeting micro RNA-103/107, as a lead compound under the agreement. In December 2015, we earned a $10.0 million clinical milestone payment upon AstraZeneca's first patient dosing in a first-in-human Phase I clinical study of RG-125(AZD4076). If RG-125(AZD4076) is successfully developed and commercialized through pre-agreed sales targets, we could receive additional milestone payments of up to $160.0 million , including up to $32.5 million for clinical milestones, and up to $127.5 million for commercialization milestones. In addition, we are entitled to receive royalties based on a percentage of net sales which will range from the mid to high single digits, depending upon the volume of sales, which royalties may be reduced in certain, limited circumstances. We evaluated the contingent event-based payments under our collaboration and license agreement with AstraZeneca and determined that the preclinical milestone and the milestone earned for the initiation of a Phase I clinical trial met the definition of substantive milestones. Accordingly, revenue for these achievements was recognized in its entirety in the period when the milestone was achieved and collectability was reasonably assured. Other contingent event-based payments under the agreement for which payment is contingent upon the results of AstraZeneca’s performance will not be accounted for using the milestone method. Such payments will be recognized as revenue over the remaining estimated period of performance, if any, and when collectability is reasonably assured. In January 2015, we entered into a letter agreement with AstraZeneca to amend the collaboration and license agreement. Under the terms of the letter agreement, we agreed to perform additional miR-103/107 program research and development activities related to RG-125(AZD4076). AstraZeneca agreed to fund 50% of the costs for these additional activities, as outlined in the letter agreement. In accordance with the collaboration and license agreement, AstraZeneca funded 100% of the costs for product manufacturing activities outlined in the letter agreement necessary to support a Phase I clinical study. In December 2015, we completed a technology transfer to AstraZeneca and have no further obligations to AstraZeneca for future development of RG-125(AZD4076). We recognized $4.5 million for the year ended December 31, 2015 for the performance of research and development and product manufacturing activities outlined in the letter agreement. As of December 31, 2015, our obligations under the letter agreement were complete. Sanofi In July 2012, we amended and restated our collaboration and license agreement with Sanofi to expand the potential therapeutic applications of the micro RNA alliance targets to be developed under such agreement. We determined that the elements within the strategic alliance agreement with Sanofi should be treated as a single unit of accounting because the delivered elements did not have stand-alone value to Sanofi. The following elements were delivered as part of the strategic alliance with Sanofi: (1) a license for up to four micro RNA targets; and (2) a research license under our technology alliance. In June 2013, the original research term expired, upon which we and Sanofi entered into an option agreement pursuant to which Sanofi was granted an exclusive right to negotiate the co-development and commercialization of certain of our unencumbered micro RNA programs and we were granted the exclusive right to negotiate with Sanofi for co-development and commercialization of certain miR-21 anti-miRs in oncology and Alport syndrome. In July 2013, we received an upfront payment of $2.5 million , of which $1.25 million is creditable against future amounts payable by Sanofi to us under any future co-development and commercialization agreement we enter pursuant to the option agreement. Revenue associated with the creditable portion of this option payment remained deferred as of December 31, 2016 , and will remain deferred until its application to a creditable transaction. The non-creditable portion of this payment, $1.25 million , was recognized as revenue over the option period from the effective date of the option agreement in June 2013 through the expiration of the option period in January 2014. In February 2014, we and Sanofi entered into a second amended and restated collaboration and license agreement (the “2014 Sanofi Amendment”) to renew our strategic alliance to discover, develop and commercialize micro RNA therapeutics to focus on specific orphan disease and oncology targets. Under the terms of our renewed alliance, Sanofi will have opt-in rights to our clinical fibrosis program targeting miR-21 for the treatment of Alport syndrome, our preclinical program targeting miR-21 for oncology indications, and our preclinical program targeting miR-221/222 for hepatocellular carcinoma (“HCC”). We are responsible for developing each of these programs to proof-of-concept, at which time Sanofi has an exclusive option on each program. If Sanofi chooses to exercise its option on any of these programs, Sanofi will reimburse us for a significant portion of our preclinical and clinical development costs and will also pay us an option exercise fee for any such program, provided that $1.25 million of the $2.5 million upfront option fee paid to us by Sanofi in connection with the June 2013 option agreement will be creditable against such option exercise fee. We are eligible to receive royalties on micro RNA therapeutic products commercialized by Sanofi and will have the right to co-promote these products. In connection with the 2014 Sanofi Amendment, we entered into a Common Stock Purchase Agreement (the “Purchase Agreement”), pursuant to which we sold 1,303,780 shares of our common stock to Aventisub LLC (formerly Aventis Holdings, Inc.) (“Aventis”), an entity affiliated with Sanofi, in a private placement at a price per share of $7.67 for an aggregate purchase price of $10.0 million . Under the terms of the Purchase Agreement, Aventis was not permitted to sell, transfer, make any short sale of, or grant any option for the sale of any common stock for the 12 -month period following its effective date. The Purchase Agreement and the 2014 Sanofi Amendment were negotiated concurrently and were therefore evaluated as a single agreement. Based upon restricted stock studies of similar duration and a Black-Scholes valuation to measure the discount for lack of marketability, approximately $0.4 million of the proceeds from the Purchase Agreement was attributed to the 2014 Sanofi Amendment, and represents consideration for the value of the program targeting miR-221/222 for HCC. As this element does not have stand-alone value, we are recognizing the $0.4 million allocated consideration into revenue ratably over the estimated period of performance of the miR-221/222 program. As of December 31, 2016 , deferred revenue associated with the Purchase Agreement and the 2014 Sanofi Amendment was $0.2 million , which we are expecting to recognize over the remaining estimated period of performance of approximately three years . We are eligible to receive milestone payments of up to $101.8 million for proof-of-concept option exercise fees (net of $1.25 million creditable, as noted above), $15.0 million for clinical milestones and up to $300.0 million for regulatory and commercial milestones. In addition, we are entitled to receive royalties based on a percentage of net sales of any products from the miR-21 and miR-221/222 programs which, in the case of sales in the United States, will be in the middle of the 10 to 20% range, and, in the case of sales outside of the United States, will range from the low end to the middle of the 10 to 20% range, depending upon the volume of sales. If we exercise our option to co-promote a product, we will continue to be eligible to receive royalties on net sales of each product in the United States at the same rate, unless we elect to share a portion of Sanofi’s profits from sales of such product in the United States in lieu of royalties. We have evaluated the contingent event-based payments under the 2014 Sanofi Amendment and determined that the milestone payments meet the definition of substantive milestones. Accordingly, revenue for these achievements will be recognized in their entirety in the period when the milestone is achieved and collectability is reasonably assured. Other contingent event-based payments under the 2014 Sanofi Amendment for which payment is contingent upon the results of Sanofi’s performance will not be accounted for using the milestone method. Such payments will be recognized as revenue over the remaining estimated period of performance, if any, and when collectability is reasonably assured. Biogen In August 2014, we entered into a collaboration and license agreement with Biogen to collaborate on micro RNA biomarkers for multiple sclerosis. Pursuant to the terms of the collaboration and license agreement, we received an upfront payment of $2.0 million , which was treated as a single unit of accounting as the license did not have stand-alone value. The $2.0 million up-front payment was recognized on a straight-line basis over the estimated period of performance of approximately one year , ending September 2015. In July 2015, the collaboration and license agreement was amended to modify the conditions of the third research-based milestone. Additionally, the amendment extended the expected research term from 12 months to 14 months . We recognized the remaining upfront payment on a straight-line basis over the amended expected term. As of December 31, 2015, our period of performance was complete and the deferred revenue balance was zero . In January 2015, May 2015, and September 2015, we earned research milestone payments under the collaboration and license agreement of $0.1 million , $0.3 million and $0.3 million , respectively. These milestone payments met the definition of substantive milestones and, accordingly, revenue for these achievements was recognized in the period the milestones were achieved and collectability was reasonably assured.

Property and Equipment, net

Property and Equipment, net	12 Months Ended
	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Property and Equipment, net	Property and Equipment, net The following table summarizes our major classes of property and equipment (in thousands): December 31, 2016 2015 Laboratory equipment $ 7,932 $ 7,310 Computer equipment and software 265 305 Furniture and fixtures 706 138 Leasehold improvements 10,469 1,930 Construction in progress 261 2,167 19,633 11,850 Less accumulated depreciation and amortization (7,803 ) (6,450 ) Property and equipment, net $ 11,830 $ 5,400 Depreciation and amortization of property and equipment of $2.2 million , $1.5 million and $1.4 million was recorded for the years ended December 31, 2016 , 2015 and 2014 , respectively.

Intangible Assets

Intangible Assets	12 Months Ended
	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets, net	Intangible Assets, net The following table summarizes our major classes of intangible assets (in thousands): December 31, 2016 2015 Patents $ 1,102 $ 1,081 Licenses 379 379 1,481 1,460 Accumulated amortization (466 ) (379 ) Intangibles, net $ 1,015 $ 1,081 Intangible asset amortization of $0.1 million was recorded for each of the years ended December 31, 2016 , 2015 and 2014 , respectively. Amortization of these intangible assets over the next five years is expected to be approximately $0.1 million per year.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Operating Lease In July 2015, we entered into a new operating lease agreement (the "Lease") for office and laboratory space in San Diego, California, for a term of 96 months from the lease commencement date, and moved the Company's headquarters into this new space in May 2016. In conjunction with the Lease, we received $1.4 million from our landlord in August 2015 for assistance with costs associated with the relocation of our corporate headquarters. These funds were received for a specific and limited purpose, and therefore were classified as restricted cash and tenant incentive obligation on the Company's balance sheet. The restricted cash balance was zero and $1.3 million as of December 31, 2016 and 2015, respectively. In addition, the Lease provided a tenant improvement allowance of up to $8.5 million , which was to be used for non-structural leasehold improvements at the new facility. The improvements at the new facility were complete in the second half of 2016, with approximately $8.2 million of the allowance having been used. Under the lease, any unused tenant improvement allowance up to $0.4 million could be used to offset future cash rent. The $0.3 million unused tenant improvement allowance was applied as a credit against future rent payments. The $1.4 million tenant incentive and $8.2 million tenant improvement allowance were classified as deferred rent on the Company's balance sheet and are amortized against rent expense on a straight-line basis over the term of the lease, which commenced upon lease inception. Previous to moving into our new Company headquarters in May 2016, the Company leased separate office and laboratory space in San Diego, California for use as the Company's corporate headquarters and research facility under an operating lease agreement ("Prior Lease"). The Prior Lease commenced in July 2010 and expires in June 2017. The Prior Lease was amended in November 2012 to increase rented space ("the Additional Space"). Early termination rights were provided with respect to a portion of the space under the Prior Lease after the 60 th month of the lease, on condition of at least 9 months prior written notice. No early termination rights were available for the Additional Space. In October 2015, we provided written notice to our landlord of our election to early terminate the Prior Lease. The effective date of termination of the Prior Lease was June 30, 2016. Termination fees associated with the early termination of the Lease were not material. The Additional Space occupied in conjunction with the amendment of the Prior Lease in November 2012 expires in June 2017. The Additional Space was sublet by the Company in August 2016 for a period extending through the June 2017 expiration of the Prior Lease term. Future sublease income of $0.2 million will be received through the expiration of this sublease agreement in June 2017. Rent expense for the years ended December 31, 2016 , 2015 and 2014 was approximately $1.2 million , $0.6 million and $0.7 million , respectively. We account for the difference between the minimum lease payments and the straight-line amount as deferred rent. Deferred rent under the Lease, which is included in other long-term liabilities, was approximately $10.1 million and zero at December 31, 2016 and 2015 , respectively. Deferred rent under the Prior Lease was zero and $0.4 million at December 31, 2016 and 2015, respectively. In connection with the exit and subsequent sublet of the Additional Space, we recorded a liability, which is included in other current liabilities, for unused office space beginning August 2016. As of December 31, 2016, unused office liability was approximately $0.2 million and will amortize against rent expense through the expiration of the Prior Lease term in June 2017. We also pay property taxes, maintenance and insurance, which are expensed as incurred. The future minimum payment summary below includes amounts payable over the remaining period of the Lease and the Prior Lease. As of December 31, 2016 , aggregate future annual minimum lease payments for our operating leases are as follows (in thousands): 2017 $ 2,330 2018 2,576 2019 2,654 2020 2,733 2021 2,815 Thereafter 6,892 $ 20,000 License Agreements We have license agreements with third parties that require us to make annual license maintenance payments and future payments upon the success of licensed products that include milestones and/or royalties. Minimum future payments over the next five years are not material.

Convertible Note Payable

Convertible Note Payable	12 Months Ended
	Dec. 31, 2016
Debt Disclosure [Abstract]
Convertible Note Payable	Convertible Note Payable In October 2012, in conjunction with our initial public offering the Post-IPO GSK Note was established in the principal amount of $5.4 million , with a maturity date of October 9, 2015 . On January 29, 2015, the principal amount outstanding under the Post-IPO GSK Note of $5.4 million was converted into 1,356,738 shares of our common stock at a conversion price of $4.00 per share. We recorded a loss from the change in valuation of the convertible note payable of zero , $1.8 million and $12.1 million on the statements of operations and comprehensive loss for the years ended December 31, 2016, 2015 and 2014, respectively.

Term Loan

Term Loan	12 Months Ended
	Dec. 31, 2016
Debt Disclosure [Abstract]
Term Loan	Term Loan On June 17, 2016, we entered into a loan and security agreement ("Loan Agreement") with Oxford Finance, LLC, ("Oxford"), pursuant to which Oxford agreed to lend us up to $30.0 million , issuable in two separate term loans of $20.0 million (the "Term A Loan") and $10.0 million (the "Term B Loan"). We collectively refer to the Term A Loan and the Term B Loan as Term Loans. On June 22, 2016, we received $20.0 million in proceeds from the Term A Loan, net of debt issuance costs. Under the terms of the Loan Agreement we may, at our sole discretion, borrow $10.0 million under the Term B Loan following the achievement of a defined milestone event until the earlier of 60 days thereafter or March 31, 2017. All outstanding Term Loans will mature on June 1, 2020 (the “Maturity Date”) and we will have interest-only payments through June 1, 2018, followed by 24 equal monthly payments of principal and unpaid accrued interest. The Term Loans will bear interest at a floating per annum rate equal to (i) 8.51% plus (ii) the greater of (a) the 30 day U.S. Dollar LIBOR rate reported in The Wall Street Journal on the last business day of the month that immediately precedes the month in which the interest will accrue and (b) 0.44% . We have the option to prepay all, but not less than all, of the borrowed amounts, provided that we will be obligated to pay a prepayment fee equal to (i) 2% of the outstanding principal balance of the applicable Term Loan if prepayment is made prior to the second anniversary of the applicable funding date of the Term Loan, provided no prepayment fee will be due in connection with a prepayment made on or prior to the first anniversary of the applicable funding date of the Term Loan in connection with an acquisition of our company, or (ii) 1% of the applicable Term Loan prepaid thereafter and prior to the Maturity Date. We will be required to make a final payment of 5.5% of the principal balance outstanding, payable on the earlier of (i) the Maturity Date, (ii) acceleration of any Term Loan, or (iii) the prepayment of the Term Loans. We may use the proceeds from the Term Loans solely for working capital and to fund our general business requirements. Our obligations under the Loan Agreement are secured by a first priority security interest in substantially all of our current and future assets, other than our intellectual property. We have also agreed not to encumber our intellectual property assets, except as permitted by the Loan Agreement. As of December 31, 2016 , we had $20.0 million outstanding under the Term A Loan. The Term A Loan was recorded at its initial carrying value of $20.0 million , less debt issuance costs of approximately $0.2 million . In connection with the Term A Loan, the debt issuance costs have been recorded as a debt discount on our balance sheets, which are being accreted to interest expense over the life of the Term A Loan using an effective interest rate of 8.98% . The exit fee is being accrued over the life of the Term A Loan through interest expense. As of December 31, 2016 , we were in compliance with all covenants under the Loan Agreement. Future principal payments for the Term A Loan due under the Loan Agreement are as follows (in thousands): 2017 $ — 2018 5,000 2019 10,000 2020 5,000 $ 20,000

Common Stock and Stockholders'

Common Stock and Stockholders' Equity	12 Months Ended
	Dec. 31, 2016
Equity [Abstract]
Common Stock and Stockholders’ Equity	Common Stock and Stockholders’ Equity Common Stock As of December 31, 2016 , there were 52,924,805 shares of common stock outstanding. Each share of common stock is entitled to one vote. The holders of the common stock are also entitled to receive dividends whenever funds are legally available and when declared by our Board of Directors. Shares Reserved for Future Issuance The following shares of common stock were reserved for future issuance as of December 31, 2016 : Common stock options outstanding 8,931,498 Common stock available for future grant under the 2012 Plan 718,299 Employee Stock Purchase Plan 1,594,452 Total common shares reserved for future issuance 11,244,249 The following table summarizes our stock option activity under all equity incentive plans for the year ended December 31, 2016 (shares and aggregate intrinsic value in thousands): Number of options Weighted average exercise price Weighted average remaining contractual term Aggregate intrinsic value Options outstanding at December 31, 2015 5,126 $ 8.91 Granted 4,966 $ 4.93 Exercised (90 ) $ 3.45 Canceled/forfeited/expired (1,071 ) $ 9.32 Options outstanding at December 31, 2016 8,931 $ 6.70 8.14 $ 34,680 Vested or expected to vest at December 31, 2016 8,278 $ 6.82 8.04 $ 31,463 Exercisable at December 31, 2016 2,882 $ 7.58 5.99 $ 9,688 The weighted average grant date fair value per share of employee stock options granted during the years ended December 31, 2016 , 2015 and 2014 was $3.47 , $7.47 and $8.37 , respectively. The total intrinsic value of stock options exercised was $0.2 million , $21.4 million and $12.1 million during the years ended December 31, 2016 , 2015 and 2014 , respectively. Cash received from the exercise of stock options was approximately $0.3 million , $6.7 million and $2.2 million for the years ended December 31, 2016 , 2015 and 2014 , respectively. Stock-Based Compensation The following table summarizes the weighted average assumptions used to estimate the fair value of stock options and performance stock awards granted to employees under our 2012 Equity Incentive Plan and 2015 Inducement Plan and the shares purchasable under our 2012 Employee Stock Purchase Plan during the periods presented: Year ended December 31, 2016 2015 2014 Stock options Risk-free interest rate 1.5 % 1.8 % 1.8 % Volatility 82.7 % 78.9 % 74.3 % Dividend yield — — — Expected term (years) 6.0 6.1 6.1 Performance stock options Risk-free interest rate 1.4 % 1.8 % 2.1 % Volatility 82.4 % 76.7 % 69.6 % Dividend yield — — — Expected term (years) 5.9 6.0 6.3 Employee stock purchase plan shares Risk-free interest rate 0.5 % 0.1 % 0.1 % Volatility 93.7 % 76.1 % 69.8 % Dividend yield — — — Expected term (years) 0.5 0.5 0.5 Risk-free interest rate - The risk-free interest rate assumption was based on observed interest rates appropriate for the expected term of the stock option grants. Expected dividend yield - The expected dividend yield assumption was based on the fact that we have never paid cash dividends and have no present intention to pay cash dividends. Expected volatility - The expected volatility assumption for the years ended December 31, 2016 and 2015 was based on the historical volatility of the trading price of our common stock. The expected volatility assumption for the year ended December 31, 2014 was based on a combination of volatility of a peer group of similar companies whose share prices were publicly available and the historical volatility of the trading price of our common stock. The peer group was developed based on companies in the biotechnology industry. Expected term - The expected term represents the period of time that options are expected to be outstanding. Because we do not have sufficient historical exercise behavior data, we determine the expected life using the simplified method, which was an average of the contractual term of the option and its ordinary vesting period. Forfeitures - We reduce stock-based compensation expense for estimated forfeitures. Forfeitures are estimated at the time of grant based upon historical information and revised, if necessary, in subsequent periods if actual forfeitures differ from those estimates. Our estimated forfeiture rates ranged from 3% to 7% for the years ended December 31, 2016 and 2015 . The following table summarizes the allocation of our stock-based compensation expense for all stock awards during the periods presented (in thousands): Year ended December 31, 2016 2015 2014 Research and development $ 5,458 $ 8,075 $ 3,939 General and administrative 7,414 7,293 3,100 Total $ 12,872 $ 15,368 $ 7,039 The total compensation cost related to non-vested awards not yet recognized was $12.3 million as of December 31, 2016 . The weighted-average period over which this expense is expected to be recognized is approximately 1.5 years. Employee Stock Purchase Plan In October 2012, we adopted the 2012 Employee Stock Purchase Plan (“2012 Purchase Plan”), which enables participants to contribute up to 15% of such participant’s eligible compensation during a defined six-month period to purchase our common stock. The purchase price of common stock under the 2012 Purchase Plan will be the lesser of: (i) 85% of the fair market value of our common stock at the inception of the enrollment period or (ii) 85% of the fair market value of our common stock at the applicable purchase date. We issued 165,684 shares of common stock under the 2012 Purchase Plan for the year ended December 31, 2016 . As of December 31, 2016 , 321,184 shares of our common stock were issued and 1,594,452 shares of our common stock were reserved for future issuance and have been authorized for purchase under the 2012 Purchase Plan. Inducement Plan On July 17, 2015, the Company’s Board of Directors adopted an Inducement Plan, which became effective immediately. Stockholder approval of the Inducement Plan was not required pursuant to Rule 5635 (c)(4) of the NASDAQ Listing Rules. The Inducement Plan initially reserved 1,000,000 shares of common stock and provides for the grant of NSOs that was used exclusively for grants to individuals that were not previously employees or directors of the Company, as an inducement material to the individual’s entry into employment with the Company. Under the Inducement Plan, options were granted with varying vesting terms, but typically vest over four years , with 25% of the total grant vesting on the first anniversary of the effective date of the option grant and the remaining grant vesting monthly thereafter over the following 36 months. All options granted under the Inducement Plan expire ten years from the date of grant. As of December 31, 2016, all shares reserved under the Inducement Plan had been granted.

Defined Contribution Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2016
Compensation and Retirement Disclosure [Abstract]
Defined Contribution Plan	Defined Contribution Plan In 2009, we established an employee 401(k) salary deferral plan (“401(k) Plan”) covering all eligible employees. Active employees who are at least 18 years old and are not otherwise disqualified under the terms of the 401(k) Plan are eligible to participate. Employees may contribute up to 50% of their compensation per year (subject to a maximum limit prescribed by federal tax law). Under the 401(k) Plan, we may elect to match a discretionary percentage of employee contributions. We have elected to match 25% of employees’ contributions up to 6% of the employees’ eligible salary. We made $0.2 million in matching contributions for the year ended December 31, 2016 and $0.1 million for each of the years ended December 31, 2015 and 2014.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The following table summarizes the components of our income tax (benefit) expense (in thousands): Year ended December 31, 2016 2015 2014 Current: Federal $ — $ — $ — State 1 1 1 1 1 1 Deferred: Federal (5 ) (17 ) — State — (2 ) — (5 ) (19 ) — Income tax (benefit) expense $ (4 ) $ (18 ) $ 1 The following is a reconciliation of the expected statutory federal income tax provision to our actual income tax provision (in thousands): Year ended December 31, 2016 2015 2014 Expected income tax benefit at federal statutory tax rate $ (27,826 ) $ (18,960 ) $ (19,271 ) State income taxes, net of federal benefit (1,152 ) (1,580 ) (2,348 ) Tax credits (5,447 ) (5,039 ) (3,307 ) Change in fair value of convertible note payable — 616 4,120 Change in valuation allowance 31,990 23,216 20,047 Prior year true-up (1,320 ) 110 (92 ) Stock compensation 2,458 1,161 902 Reserve for uncertain tax positions 1,314 254 — Other (21 ) 204 (50 ) Income tax (benefit) expense $ (4 ) $ (18 ) $ 1 The following table summarizes the significant components of our deferred tax assets and liabilities (in thousands): December 31, 2016 2015 Deferred tax assets: Net operating loss carryovers $ 70,739 $ 48,912 Research and development and other tax credits 22,613 15,148 Deferred revenue 735 1,210 Intangibles and property and equipment basis difference — 2,097 Stock compensation expense 5,664 3,488 Other 4,679 1,129 Total deferred tax assets 104,430 71,984 Total deferred tax liabilities (799 ) (343 ) Net deferred tax asset 103,631 71,641 Valuation allowance (103,631 ) (71,641 ) Net deferred tax asset $ — $ — For all periods presented, we have determined that it is more likely than not that our deferred tax asset will not be realized. Accordingly, we have recorded a valuation allowance to fully offset the net deferred tax asset of $103.6 million . As of December 31, 2016, we had federal and California tax net operating loss carryforwards of $204.8 million and $173.2 million , respectively, which begin to expire in 2030 and 2031. At December 31, 2016, approximately $19.4 million and $19.4 million of the Federal and State net operating loss carryforwards, respectively, relate to stock option exercises, which will result in an increase to additional paid-in capital and a decrease in income taxes payable at the time when the tax loss carryforwards are utilized. Upon adoption of ASU 2016-09 in the first quarter of 2017, the balance of the unrecognized excess tax benefits will be reversed with the impact recorded to accumulated deficit, including any change to the valuation allowance as a result of the adoption. Due to the full valuation allowance on the U.S. deferred tax assets as of December 31, 2016, we do not expect any impact to the financial statements as a result of this adoption. As of December 31, 2016, we also had federal and California research and development tax credit carryforwards of $20.3 million and $6.9 million , respectively. The federal research and development tax credit carryforwards will begin to expire in 2029. The California research and development tax credit carryforwards are available indefinitely. As of December 31, 2016, we also had $0.2 million of Federal Alternative Minimum Tax Credit carryforwards that are available indefinitely. The future utilization of our research and development credit carryforwards and net operating loss carryforwards to offset future taxable income may be subject to an annual limitation as a result of ownership changes that have occurred previously or may occur in the future. The Tax Reform Act of 1986 (the Act) limits a company’s ability to utilize certain tax credit carryforwards and net operating loss carryforwards in the event of a cumulative change in ownerships in excess of 50% as defined in the Act. The following table summarizes the changes in the amount of our unrecognized tax benefits (in thousands): Year Ended December 31, 2016 2015 2014 Beginning balance of unrecognized tax benefits $ 2,298 $ 1,853 $ 1,092 Increase (decrease) for prior year tax positions 1,991 (250 ) (73 ) Increase for current year tax positions 1,343 695 834 Total $ 5,632 $ 2,298 $ 1,853 Included in unrecognized tax benefits of $5.6 million at December 31, 2016 was $3.7 million of tax benefits that, if recognized, would reduce our annual effective tax rate, subject to valuation allowance. We are subject to taxation in the United States and California. Our tax years for 2009 and forward are subject to examination by the U.S. tax authorities and our tax years for 2009 and forward are subject to examination by the California tax authorities due to carryforward of unutilized net operating losses and research and development credits. It is our practice to recognize interest and/or penalties related to income tax matters in income tax expense. For the years ended December 31, 2016 , 2015 and 2014 , we have not recognized any interest or penalties related to income taxes.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2016
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions In September 2014, we entered into an agreement with Sanofi-Aventis Deutschland GmbH (“Sanofi Deutschland”), a contract manufacturing subsidiary of Sanofi, for the manufacture of certain drug substance requirements and other services to support our preclinical and clinical activities associated with the RG-012 program. Pursuant to this agreement, we may engage Sanofi Deutschland from time-to-time to manufacture RG-012 drug product on our behalf. Expenses incurred under the Sanofi agreement for services performed or out-of-pocket expenses were $0.9 million , $0.4 million and zero for the years ended December 31, 2016 , 2015 and 2014 , respectively. In February 2015, we entered into a letter agreement with Alnylam Pharmaceuticals, Inc. ("Alnylam") pursuant to which we and Alnylam agreed to the financial terms for certain technology acquired by Alnylam within the licensed patent rights under our Amended and Restated License and Collaboration Agreement (the “Additional Patent Rights”) with Alnylam and Ionis. In addition to any royalties payable by us to Alnylam pursuant to the terms of the Amended and Restated License and Collaboration Agreement, we agreed to pay Alnylam an additional low single-digit royalty on net sales of certain products utilizing the Additional Patent Rights, with the exact royalty percentage payable being dependent on the total amount of net sales during the calendar year. We also agreed to pay Alnylam milestone payments on certain products utilizing the additional patent rights of up to $33.0 million per product upon the achievement of certain regulatory milestone events. There was no activity under this agreement for the years ended December 31, 2016 and 2015.

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data (Unaudited)	Selected Quarterly Financial Data (Unaudited) The following financial information reflects all normal recurring adjustments, which are, in the opinion of management, necessary for a fair statement of the results of the interim periods. Summarized quarterly data for 2016 and 2015 are as follows (in thousands, except per share data): For the quarters ending March 31 June 30 September 30 December 31 2016 Revenues $ 489 $ 483 $ 204 $ 18 Operating expenses 21,867 21,671 19,396 19,762 Net loss (21,207 ) (21,090 ) (19,519 ) (20,020 ) Basic and diluted net loss per share $ (0.40 ) $ (0.40 ) $ (0.37 ) $ (0.38 ) 2015 Revenues $ 4,200 $ 3,834 $ 1,865 $ 10,860 Operating expenses 17,071 25,015 15,210 18,221 Net loss (14,487 ) (21,035 ) (13,000 ) (7,226 ) Basic net loss per share(1) $ (0.29 ) $ (0.41 ) $ (0.25 ) $ (0.14 ) (1) Net loss per share is computed independently for each of the quarters presented. Therefore, the sum of the quarterly per-share calculations will not necessarily equal the annual per share calculation.

The Business, Basis of Presen23

The Business, Basis of Presentation and Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates Our financial statements are prepared in accordance with United States Generally Accepted Accounting Principles, which requires us to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses and the disclosure of contingent assets and liabilities in our financial statements and accompanying notes. An estimated loss contingency is accrued in our financial statements if it is probable that a liability has been incurred and the amount of the loss can be reasonably estimated. Although these estimates are based on our knowledge of current events and actions we may undertake in the future, actual results may ultimately differ from these estimates and assumptions.
Revenue Recognition	Revenue Recognition Our revenues generally consist of upfront payments for licenses or options to obtain licenses in the future, milestone payments and payments for other research services under strategic alliance and collaboration agreements. We recognize revenues when all four of the following criteria are met: (1) persuasive evidence of an arrangement exists; (2) delivery of the products and/or services has occurred; (3) the selling price is fixed or determinable; and (4) collectability is reasonably assured. Multiple element arrangements, such as our strategic alliance agreements with Sanofi and AstraZeneca AB (“AstraZeneca”), are analyzed to determine whether the deliverables within the agreement can be separated or whether they must be accounted for as a single unit of accounting. Deliverables under the agreement will be accounted for as separate units of accounting provided that (i) a delivered item has value to the customer on a stand-alone basis; and (ii) if the agreement includes a general right of return relative to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in the control of the vendor. The allocation of consideration amongst the deliverables under the agreement is derived using a “best estimate of selling price” if vendor specific objective evidence and third-party evidence of fair value is not available. If the delivered element does not have stand-alone value, the arrangement is then accounted for as a single unit of accounting, and we recognize the consideration received under the arrangement as revenue on a straight-line basis, which approximates effort over our estimated period of performance, which for us is typically the expected term of the research and development plan. Milestones We apply the milestone method of accounting to recognize revenue from milestone payments when earned, as evidenced by written acknowledgement from the collaborator or other persuasive evidence that the milestone has been achieved and the payment is non-refundable, provided that the milestone event is substantive. A milestone event is defined as an event (i) that can only be achieved based in whole or in part on either our performance or on the occurrence of a specific outcome resulting from our performance; (ii) for which there is substantive uncertainty at the inception of the arrangement that the event will be achieved; and (iii) that would result in additional payments being due to us. Events for which the occurrence is either contingent solely upon the passage of time or the result of a counterparty’s performance are not considered to be milestone events. A milestone event is substantive if all of the following conditions are met: (i) the consideration is commensurate with either our performance to achieve the milestone, or the enhancement of the value to the delivered item(s) as a result of a specific outcome resulting from our performance to achieve the milestone; (ii) the consideration relates solely to past performance; and (iii) the consideration is reasonable relative to all the deliverables and payment terms (including other potential milestone consideration) within the arrangement. We assess whether a milestone is substantive at the inception of each arrangement. If a milestone is deemed non-substantive, we will account for that milestone payment using a method consistent with the related units of accounting for the arrangement over the estimated performance period. Deferred Revenue Amounts received prior to satisfying the above revenue recognition criteria are recorded as deferred revenue in the accompanying balance sheets. Amounts not expected to be recognized within the next 12 months are classified as non-current deferred revenue.
Stock-Based Compensation	Stock-Based Compensation We account for stock-based compensation expense related to stock options granted to employees and members of our board of directors by estimating the fair value of each stock option on the date of grant using the Black-Scholes option pricing model. We recognize stock-based compensation expense using the accelerated multiple-option approach. Under the accelerated multiple-option approach (also known as the graded-vesting method), we recognize compensation expense over the requisite service period for each separately vesting tranche of the award as though the award was in substance multiple awards, resulting in accelerated expense recognition over the vesting period. For performance-based awards granted to employees (i) the fair value of the award is determined on the grant date, (ii) we assess the probability of the individual milestones under the award being achieved and (iii) the fair value of the shares subject to the milestone is expensed over the implicit service period commencing once management believes the performance criteria is probable of being met. We account for stock options granted to non-employees using the fair value approach. Stock options granted to non-employees are subject to periodic revaluation over their vesting terms.
Prepaid Materials	Prepaid Materials We capitalize the purchase of certain raw materials and related supplies for use in the manufacturing of drug product in our clinical development programs, as we have determined that these materials have alternative future use. We can use these raw materials and related supplies in multiple clinical drug products, and therefore have future use independent of the development status of any particular drug program until it is utilized in the manufacturing process. We periodically review these capitalized materials for indicators of impairment, including shelf life, continued alternative future use and obsolescence.
Research and Development	Research and Development Research and development costs are expensed as incurred and consist of costs associated with research activities supporting our drug discovery efforts, compensation and related benefits, non-cash stock-based compensation, license fees, laboratory supplies and associated overhead and facility costs.
Income Tax, Policy	Income Taxes Income taxes are accounted for under the asset and liability method. This approach requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of the differences between the tax basis of assets or liabilities and their carrying amounts in the financial statements using the enacted tax rates and laws that are anticipated to be in effect when the differences are expected to reverse. We provide a valuation allowance against net deferred tax assets if it is more likely than not that these items will either expire before the Company is able to realize their benefit or if future deductibility is uncertain. In accordance with the accounting standards for uncertain tax positions, the Company evaluates the recognition threshold and measurement attribute criteria for the financial statement recognition and measurement of tax positions taken or expected to be taken in a tax return. For those benefits to be recognized, a tax position must be more likely than not to be sustained upon examination by taxing authorities.
Fair Value Option	Fair Value Option Applicable accounting policies permit entities to choose, at specified election dates, to measure specified items at fair value if the decision about the election is: (1) applied instrument by instrument, (2) irrevocable, and (3) applied to an entire instrument. Fair Value Measurements We have certain financial assets and liabilities recorded at fair value which have been classified as Level 1, 2, or 3 within the fair value hierarchy as described in the accounting standards for fair value measurements. Accounting standards define fair value as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants as of the measurement date. Market participants are buyers and sellers in the principal market that are (i) independent, (ii) knowledgeable, (iii) able to transact, and (iv) willing to transact. The accounting standards provide an established hierarchy for inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the most observable inputs be used when available. Observable inputs are inputs that market participants would use in valuing the asset or liability and are developed based on market data obtained from independent sources. Unobservable inputs are inputs that reflect our assumptions about the factors that market participants would use in valuing the asset or liability. The accounting standards prioritize the inputs used in measuring the fair value into the following hierarchy: • Level 1 includes financial instruments for which quoted market prices for identical instruments are available in active markets. • Level 2 includes financial instruments for which there are inputs other than quoted prices included within Level 1 that are observable for the instrument such as quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets with insufficient volume or infrequent transactions (less active markets) or model-driven valuations in which significant inputs are observable or can be derived principally from, or corroborated by, observable market data. • Level 3 includes financial instruments for which fair value is derived from valuation techniques in which one or more significant inputs are unobservable, including management’s own assumptions.
Clinical Trial and Preclinical Study Accruals	Clinical Trial and Preclinical Study Accruals We make estimates of our accrued expenses for clinical trial and preclinical study activities as of each balance sheet date in our financial statements based on the facts and circumstances known to us at that time. These accruals are based upon estimates of costs incurred and fees that may be associated with services provided by clinical trial investigational sites, clinical research organizations (“CROs”) and for other clinical trial-related activities. Payments under certain contracts with such parties depend on factors such as successful enrollment of patients, site initiation and the completion of clinical trial milestones. In accruing for these services, we estimate the time period over which services will be performed and the level of effort to be expended in each period. If possible, we obtain information regarding unbilled services directly from these service providers. However, we may be required to estimate these services based on other information available to us. If we underestimate or overestimate the activities or fees associated with a study or service at a given point in time, adjustments to research and development expenses may be necessary in future periods. Historically, our estimated accrued liabilities have approximated actual expense incurred. Subsequent changes in estimates may result in a material change in our accruals.
Cash and Cash Equivalents	Cash and Cash Equivalents We classify time deposits and other investments that are highly liquid and have maturities of 90 days or less at the date of purchase as cash equivalents. The carrying amounts approximate fair value due to the short maturities of these instruments.
Restricted Cash	Restricted Cash Restricted cash consists of amounts received for a specific and limited purpose, and therefore not available for general operating activities.
Short-Term Investments	Short-Term Investments We carry short-term investments classified as available-for-sale at fair value as determined by prices for identical or similar securities at the balance sheet date. Our short-term investments consist of both Level 1 and Level 2 financial instruments in the fair value hierarchy. We record unrealized gains and losses as a component of other comprehensive loss within the statements of operations and comprehensive loss and as a separate component of stockholders’ equity. We determine the realized gains or losses of available-for-sale securities using the specific identification method and include net realized gains and losses in interest income. At each balance sheet date, we assess available-for-sale securities in an unrealized loss position to determine whether the unrealized loss is other-than-temporary. We consider factors including: the significance of the decline in value compared to the cost basis, underlying factors contributing to a decline in the prices of securities in a single asset class, the length of time the market value of the security has been less than its cost basis, the security’s relative performance versus its peers, sector or asset class, expected market volatility and the market and economy in general. When we determine that a decline in the fair value below its cost basis is other-than-temporary, we recognize an impairment loss in the year in which the other-than-temporary decline occurred. We determined that there were no other-than-temporary declines in the value of short-term investments as of December 31, 2016 or 2015 .
Concentration of Credit Risk	Concentrations of Credit Risk Financial instruments that potentially subject us to significant concentrations of credit risk consist primarily of cash equivalents and short-term investments. We maintain deposits in federally insured financial institutions in excess of federally insured limits. We have not experienced any material losses in such accounts and believe we are not exposed to significant risk. We maintain our cash equivalents and short-term investments with two highly accredited financial institutions. We invest our excess cash primarily in commercial paper, certificates of deposit and debt instruments of financial institutions and corporations, United States Treasury securities and United States government-sponsored enterprise securities. Additionally, we adhere to established guidelines regarding approved investments and maturities of investments, which are designed to preserve their principal value and maintain liquidity.
Property and Equipment	Property and Equipment We carry our property and equipment at cost, which consists of lab equipment, computer equipment and software, furniture and fixtures and leasehold improvements. Property and equipment is depreciated using the straight-line method over the estimated useful lives (generally three to five years). Leasehold improvements are amortized over the lesser of their useful life or the remaining lease term, including any renewal periods that are deemed to be reasonably assured. Repair and maintenance costs that do not improve service potential or extend economic life are expensed as incurred.
Intangibles	Intangibles We capitalize costs which consist principally of outside legal costs and filing fees related to obtaining patents. We review our capitalized patent costs periodically to determine that they include costs for patent applications that have future value and an alternative future use. We evaluate costs related to patents that we are not actively pursuing and write off these costs. We amortize patent costs over their patent lives, beginning with the date the patents are issued. The weighted average remaining life of the issued patents was approximately 10 years at December 31, 2016 . We obtain licenses from third parties and capitalize the costs related to exclusive licenses that have alternative future use within multiple potential programs. We amortize capitalized licenses over their estimated useful life or term of the agreement, which for current licenses is 10 years.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets We regularly review the carrying amount of our property, equipment and intangible assets to determine whether indicators of impairment may exist which warrant adjustments to carrying values or estimated useful lives. If indications of impairment exist, projected future undiscounted cash flows associated with the asset are compared to the carrying amount to determine whether the asset’s value is recoverable. If the carrying value of the asset exceeds such projected undiscounted cash flows, the asset will be written down to its estimated fair value.
Segment Reporting	Segment Reporting Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker in making decisions regarding resource allocation and assessing performance. To date, we have viewed our operations and managed our business as one segment operating primarily within the United States.
Comprehensive Loss	Comprehensive Loss Comprehensive loss is defined as the change in equity during a period from transactions and other events and/or circumstances from non-owner sources. Our only component of other comprehensive loss is unrealized gains (losses) on available-for-sale securities. Comprehensive gains (losses) have been reflected in the statements of operations and comprehensive loss and as a separate component in the statements of stockholders’ equity for all periods presented.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2014-09, Revenue from Contracts with Customers (Topic 606) , which outlines a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and supersedes most current revenue recognition guidance, including industry-specific guidance. ASU 2014-09 outlines a five-step process for revenue recognition that focuses on transfer of control, as opposed to transfer of risk and rewards, and also requires enhanced disclosures regarding the nature, amount, timing and uncertainty of revenues and cash flows from contracts with customers. Major provisions include determining which goods and services are distinct and require separate accounting (performance obligations), how variable consideration (which may include change orders and claims) is recognized, whether revenue should be recognized at a point in time or over time and ensuring the time value of money is considered in the transaction price. The FASB issued supplemental adoption guidance and clarification to ASU 2014-09 in March 2016, April 2016 and May 2016 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 and ASU 2016-12, Revenue from Contracts with Customers: Narrow-Scope Improvements and Practical Expedients , respectively. ASU 2014-09 and the related supplemental ASUs are effective for fiscal years beginning after December 15, 2017 and interim periods therein. The ASU permits two methods of adoption: the full retrospective method or the modified retrospective method. We currently plan on applying the modified retrospective method upon adoption in the first quarter of 2018, and currently do not anticipate that the adoption of this ASU will have a material impact with regard to our current contracts. In August 2014, the FASB issued ASU No. 2014-15, Presentation of Financial Statements Going Concern , which requires management to assess an entity’s ability to continue as a going concern, and to provide related footnote disclosure in certain circumstances. This standard is effective for annual reporting periods ending after December 15, 2016 and interim periods thereafter. The adoption of this guidance had no impact on our financial statements. In April 2015, the FASB issued ASU No. 2015-03, Interest- Imputation of Interest: Simplifying the Presentation of Debt Issuance Costs, which requires that debt issuance costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from the carrying amount of that debt liability, consistent with debt discounts. The recognition and measurement guidance for debt issuance costs were not affected by the amendments in ASU No. 2015-03. In June 2016, upon entering into a loan and security agreement, we adopted ASU No. 2015-03, which resulted in the classification of $0.2 million of debt issuance costs against the principal balance of our outstanding term loan of $20.0 million . In January 2016, the FASB issued ASU No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities , which eliminates the requirement for public companies to disclose the method(s) and significant assumptions used to estimate the fair value that is required to be disclosed for financial instruments measured at amortized cost on the balance sheet. Additionally, the standard requires public companies to use the exit price notion when measuring the fair value of financial instruments for disclosure purposes. Furthermore, the standard requires presentation of financial assets and liabilities by measurement category and form of financial asset on the balance sheet or accompanying notes to the financial statements. The standard is effective for annual reporting periods beginning after December 15, 2017, including interim periods within those annual reporting periods. Early application is permitted. The adoption of this guidance will have no impact on our financial statements. In February 2016, the FASB issued ASU No. 2016-02, Leases, which increases transparency and comparability among organizations by requiring recognition of lease assets and lease liabilities on the balance sheet and disclosure of key information about leasing arrangements. The standard is effective for annual reporting periods beginning after December 15, 2018, including interim periods within those annual reporting periods. Early application is permitted. We are currently evaluating the impact of adoption on our financial statements. In March 2016, the FASB issued ASU No. 2016-09, Compensation - Stock Compensation: Improvements to Employee Share-Based Payment Accounting, which is intended to simplify several aspects of accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. The standard is effective for annual reporting periods beginning after December 15, 2016, and interim periods within those annual reporting periods. Early application is permitted. We intend to adopt this guidance in the first quarter of 2017. We expect that the adoption of this guidance will increase our deferred tax assets with a corresponding increase to our valuation allowance. We maintained a full valuation allowance against our deferred tax assets at December 31, 2016. In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments , which addresses the presentation and classification of certain cash receipts and cash payments in the statement of cash flows under Accounting Standards Codification 230. The standard is effective for annual reporting periods beginning after December 15, 2017, and interim periods within those annual reporting periods. Early application is permitted. The adoption of this guidance will have no impact on our financial statements. In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows: Restricted Cash , which requires restricted cash and restricted cash equivalents to be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. The standard is effective for annual reporting periods beginning after December 15, 2017, and interim periods within those annual reporting periods. Early application is permitted. We have not elected to early adopt this standard as of December 31, 2016. Upon adoption, the 2016 period in our three-year statements of cash flows will include $1.3 million of restricted cash in cash and cash equivalents at the beginning of the period. We do not expect any additional impact on our financial statements.

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2016
Earnings Per Share [Abstract]
Potentially Dilutive Securities Not Included in Calculation of Diluted Net Loss per Share	Potentially dilutive securities not included in the calculation of diluted net loss per share because to do so would be anti-dilutive are as follows (in common equivalent shares): Years ended December 31, 2016 2015 2014 Common stock options 5,553,210 1,674,119 2,566,423 Convertible note payable — — 1,461,474 Total 5,553,210 1,674,119 4,027,897

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Summary of Short-Term Investments	The following tables summarize our short-term investments (in thousands): Maturity (in years) Amortized cost Unrealized Estimated fair value Gains Losses As of December 31, 2016 Corporate debt securities 2 or less $ 49,185 $ 12 $ (77 ) $ 49,120 Certificates of deposit 1 or less 9,291 — — 9,291 Commercial paper 1 or less 1,247 — — 1,247 U.S. treasury securities 1 or less 1,001 — (1 ) 1,000 Debt securities of U.S. government-sponsored agencies 1 or less 512 — — 512 Total $ 61,236 $ 12 $ (78 ) $ 61,170 Maturity (in years) Amortized cost Unrealized Estimated fair value Gains Losses As of December 31, 2015 Corporate debt securities 2 or less $ 81,054 $ 16 $ (103 ) $ 80,967 Certificates of deposit 2 or less 13,640 — — 13,640 Commercial paper 1 or less 3,490 6 — 3,496 Total $ 98,184 $ 22 $ (103 ) $ 98,103

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Fair Value Hierarchy for Assets and Liabilities Measured at Fair Value on Recurring Basis	The following table presents our fair value hierarchy for assets and liabilities measured at fair value on a recurring basis as of December 31, 2016 and December 31, 2015 (in thousands): Fair value as of December 31, 2016 Total Level 1 Level 2 Level 3 Assets: Cash equivalents $ 13,578 $ 13,578 $ — $ — Corporate debt securities 49,120 — 49,120 — Certificates of deposit 9,291 — 9,291 — Commercial paper 1,247 — 1,247 — U.S. treasury securities 1,000 — 1,000 — Debt securities of U.S. government-sponsored agencies 512 — 512 — $ 74,748 $ 13,578 $ 61,170 $ — Fair value as of December 31, 2015 Total Level 1 Level 2 Level 3 Assets: Cash equivalents $ 15,152 $ 15,152 $ — $ — Corporate debt securities 80,967 — 80,967 — Certificates of deposit 13,640 — 13,640 — Commercial paper 3,496 — 3,496 — $ 113,255 $ 15,152 $ 98,103 $ —

Strategic Alliances and Colla27

Strategic Alliances and Collaborations (Tables)	12 Months Ended
	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Revenue from Strategic Alliances	The following table summarizes our total revenues from our strategic alliances and collaborations during the periods presented (in thousands): Year ended December 31, 2016 2015 2014 AstraZeneca $ 1,122 $ 18,871 $ 1,859 Sanofi 72 71 979 GSK — — 3,509 Biogen — 1,817 1,122 Other — — 200 Total $ 1,194 $ 20,759 $ 7,669

Property and Equipment, net (Ta

Property and Equipment, net (Tables)	12 Months Ended
	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	The following table summarizes our major classes of property and equipment (in thousands): December 31, 2016 2015 Laboratory equipment $ 7,932 $ 7,310 Computer equipment and software 265 305 Furniture and fixtures 706 138 Leasehold improvements 10,469 1,930 Construction in progress 261 2,167 19,633 11,850 Less accumulated depreciation and amortization (7,803 ) (6,450 ) Property and equipment, net $ 11,830 $ 5,400

Intangible Assets (Tables)

Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Finite-Lived Intangible Assets	The following table summarizes our major classes of intangible assets (in thousands): December 31, 2016 2015 Patents $ 1,102 $ 1,081 Licenses 379 379 1,481 1,460 Accumulated amortization (466 ) (379 ) Intangibles, net $ 1,015 $ 1,081

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Rental Payments for Operating Leases	As of December 31, 2016 , aggregate future annual minimum lease payments for our operating leases are as follows (in thousands): 2017 $ 2,330 2018 2,576 2019 2,654 2020 2,733 2021 2,815 Thereafter 6,892 $ 20,000

Term Loan (Tables)

Term Loan (Tables)	12 Months Ended
	Dec. 31, 2016
Debt Disclosure [Abstract]
Schedule of Future Principal Payments	Future principal payments for the Term A Loan due under the Loan Agreement are as follows (in thousands): 2017 $ — 2018 5,000 2019 10,000 2020 5,000 $ 20,000

Common Stock and Stockholders32

Common Stock and Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2016
Equity [Abstract]
Common Stock Reserved for Future Issuance	The following shares of common stock were reserved for future issuance as of December 31, 2016 : Common stock options outstanding 8,931,498 Common stock available for future grant under the 2012 Plan 718,299 Employee Stock Purchase Plan 1,594,452 Total common shares reserved for future issuance 11,244,249
Stock Option Activity	The following table summarizes our stock option activity under all equity incentive plans for the year ended December 31, 2016 (shares and aggregate intrinsic value in thousands): Number of options Weighted average exercise price Weighted average remaining contractual term Aggregate intrinsic value Options outstanding at December 31, 2015 5,126 $ 8.91 Granted 4,966 $ 4.93 Exercised (90 ) $ 3.45 Canceled/forfeited/expired (1,071 ) $ 9.32 Options outstanding at December 31, 2016 8,931 $ 6.70 8.14 $ 34,680 Vested or expected to vest at December 31, 2016 8,278 $ 6.82 8.04 $ 31,463 Exercisable at December 31, 2016 2,882 $ 7.58 5.99 $ 9,688
Assumptions Used to Estimate Fair Value of Stock Options and Performance Stock and Employee Stock Purchase Plan	The following table summarizes the weighted average assumptions used to estimate the fair value of stock options and performance stock awards granted to employees under our 2012 Equity Incentive Plan and 2015 Inducement Plan and the shares purchasable under our 2012 Employee Stock Purchase Plan during the periods presented: Year ended December 31, 2016 2015 2014 Stock options Risk-free interest rate 1.5 % 1.8 % 1.8 % Volatility 82.7 % 78.9 % 74.3 % Dividend yield — — — Expected term (years) 6.0 6.1 6.1 Performance stock options Risk-free interest rate 1.4 % 1.8 % 2.1 % Volatility 82.4 % 76.7 % 69.6 % Dividend yield — — — Expected term (years) 5.9 6.0 6.3 Employee stock purchase plan shares Risk-free interest rate 0.5 % 0.1 % 0.1 % Volatility 93.7 % 76.1 % 69.8 % Dividend yield — — — Expected term (years) 0.5 0.5 0.5
Stock-Based Compensation	The following table summarizes the allocation of our stock-based compensation expense for all stock awards during the periods presented (in thousands): Year ended December 31, 2016 2015 2014 Research and development $ 5,458 $ 8,075 $ 3,939 General and administrative 7,414 7,293 3,100 Total $ 12,872 $ 15,368 $ 7,039

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Schedule of Components of Income Tax Expense (Benefit)	The following table summarizes the components of our income tax (benefit) expense (in thousands): Year ended December 31, 2016 2015 2014 Current: Federal $ — $ — $ — State 1 1 1 1 1 1 Deferred: Federal (5 ) (17 ) — State — (2 ) — (5 ) (19 ) — Income tax (benefit) expense $ (4 ) $ (18 ) $ 1
Schedule of Effective Income Tax Rate Reconciliation	The following is a reconciliation of the expected statutory federal income tax provision to our actual income tax provision (in thousands): Year ended December 31, 2016 2015 2014 Expected income tax benefit at federal statutory tax rate $ (27,826 ) $ (18,960 ) $ (19,271 ) State income taxes, net of federal benefit (1,152 ) (1,580 ) (2,348 ) Tax credits (5,447 ) (5,039 ) (3,307 ) Change in fair value of convertible note payable — 616 4,120 Change in valuation allowance 31,990 23,216 20,047 Prior year true-up (1,320 ) 110 (92 ) Stock compensation 2,458 1,161 902 Reserve for uncertain tax positions 1,314 254 — Other (21 ) 204 (50 ) Income tax (benefit) expense $ (4 ) $ (18 ) $ 1
Schedule of Deferred Tax Assets and Liabilities	The following table summarizes the significant components of our deferred tax assets and liabilities (in thousands): December 31, 2016 2015 Deferred tax assets: Net operating loss carryovers $ 70,739 $ 48,912 Research and development and other tax credits 22,613 15,148 Deferred revenue 735 1,210 Intangibles and property and equipment basis difference — 2,097 Stock compensation expense 5,664 3,488 Other 4,679 1,129 Total deferred tax assets 104,430 71,984 Total deferred tax liabilities (799 ) (343 ) Net deferred tax asset 103,631 71,641 Valuation allowance (103,631 ) (71,641 ) Net deferred tax asset $ — $ —
Summary of Income Tax Contingencies	The following table summarizes the changes in the amount of our unrecognized tax benefits (in thousands): Year Ended December 31, 2016 2015 2014 Beginning balance of unrecognized tax benefits $ 2,298 $ 1,853 $ 1,092 Increase (decrease) for prior year tax positions 1,991 (250 ) (73 ) Increase for current year tax positions 1,343 695 834 Total $ 5,632 $ 2,298 $ 1,853

Selected Quarterly Financial 34

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Information	Summarized quarterly data for 2016 and 2015 are as follows (in thousands, except per share data): For the quarters ending March 31 June 30 September 30 December 31 2016 Revenues $ 489 $ 483 $ 204 $ 18 Operating expenses 21,867 21,671 19,396 19,762 Net loss (21,207 ) (21,090 ) (19,519 ) (20,020 ) Basic and diluted net loss per share $ (0.40 ) $ (0.40 ) $ (0.37 ) $ (0.38 ) 2015 Revenues $ 4,200 $ 3,834 $ 1,865 $ 10,860 Operating expenses 17,071 25,015 15,210 18,221 Net loss (14,487 ) (21,035 ) (13,000 ) (7,226 ) Basic net loss per share(1) $ (0.29 ) $ (0.41 ) $ (0.25 ) $ (0.14 ) (1) Net loss per share is computed independently for each of the quarters presented. Therefore, the sum of the quarterly per-share calculations will not necessarily equal the annual per share calculation.

The Business, Basis of Presen35

The Business, Basis of Presentation and Summary of Significant Accounting Policies - Narrative (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015	Aug. 31, 2015
Accounting Policies [Abstract]
Prepaid materials	$ 5,600,000	$ 5,500,000
Prepaid expenses reserve	600,000
Restricted cash	$ 0	$ 1,256,000	$ 1,400,000

The Business, Basis of Presen36

The Business, Basis of Presentation and Summary of Significant Accounting Policies - Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2016
Minimum
Property, Plant and Equipment [Line Items]
Useful life, in years	3 years
Maximum
Property, Plant and Equipment [Line Items]
Useful life, in years	5 years

The Business, Basis of Presen37

The Business, Basis of Presentation and Summary of Significant Accounting Policies - Intangibles (Details)	12 Months Ended
	Dec. 31, 2016
Licenses
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life	10 years
Weighted Average | Patents
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life	10 years

The Business, Basis of Presen38

The Business, Basis of Presentation and Summary of Significant Accounting Policies - Segment Reporting (Details)	12 Months Ended
	Dec. 31, 2016segment
Accounting Policies [Abstract]
Number of operating segments	1

The Business, Basis of Presen39

The Business, Basis of Presentation and Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Details) - USD ($)	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015	Aug. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Restricted cash	$ 0		$ 1,256,000	$ 1,400,000
Debt Financing Agreement
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred finance costs	200,000	$ 200,000
Debt instrument, face amount		$ 20,000,000
Accounting Standard Update 2016-18
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Restricted cash	$ 1,300,000

Net Loss Per Share - Potentiall

Net Loss Per Share - Potentially Dilutive Securities Not Included in Calculation of Diluted Net Loss per Share (Details) - shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities not included in calculation of diluted net loss per share (in shares)	5,553,210	1,674,119	4,027,897
Convertible note payable
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities not included in calculation of diluted net loss per share (in shares)	0	0	1,461,474
Common stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities not included in calculation of diluted net loss per share (in shares)	5,553,210	1,674,119	2,566,423

Investments - Summary of Short-

Investments - Summary of Short-Term Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Investment [Line Items]
Maturity weighted average period, in years (less than)	2 years
Amortized cost	$ 61,236	$ 98,184
Unrealized Gains	12	22
Unrealized Losses	(78)	(103)
Estimated fair value	$ 61,170	$ 98,103
Corporate debt securities
Investment [Line Items]
Maturity (or less) (in years)	2 years	2 years
Amortized cost	$ 49,185	$ 81,054
Unrealized Gains	12	16
Unrealized Losses	(77)	(103)
Estimated fair value	$ 49,120	$ 80,967
Certificates of deposit
Investment [Line Items]
Maturity (or less) (in years)	1 year	2 years
Amortized cost	$ 9,291	$ 13,640
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated fair value	$ 9,291	$ 13,640
Commercial paper
Investment [Line Items]
Maturity (or less) (in years)	1 year	1 year
Amortized cost	$ 1,247	$ 3,490
Unrealized Gains	0	6
Unrealized Losses	0	0
Estimated fair value	$ 1,247	$ 3,496
U.S. treasury securities
Investment [Line Items]
Maturity (or less) (in years)	1 year
Amortized cost	$ 1,001
Unrealized Gains	0
Unrealized Losses	(1)
Estimated fair value	$ 1,000
Debt securities of U.S. government-sponsored agencies
Investment [Line Items]
Maturity (or less) (in years)	1 year
Amortized cost	$ 512
Unrealized Gains	0
Unrealized Losses	0
Estimated fair value	$ 512

Fair Value Measurements - Fair

Fair Value Measurements - Fair Value Hierarchy for Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	$ 74,748	$ 113,255
Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	13,578	15,152
Corporate debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	49,120	80,967
Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	9,291	13,640
Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	1,247	3,496
U.S. treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	1,000
Debt securities of U.S. government-sponsored agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	512
Fair Value, Inputs, Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	13,578	15,152
Fair Value, Inputs, Level 1 | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	13,578	15,152
Fair Value, Inputs, Level 1 | Corporate debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	0	0
Fair Value, Inputs, Level 1 | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	0	0
Fair Value, Inputs, Level 1 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	0	0
Fair Value, Inputs, Level 1 | U.S. treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	0
Fair Value, Inputs, Level 1 | Debt securities of U.S. government-sponsored agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	0
Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	61,170	98,103
Fair Value, Inputs, Level 2 | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	0	0
Fair Value, Inputs, Level 2 | Corporate debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	49,120	80,967
Fair Value, Inputs, Level 2 | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	9,291	13,640
Fair Value, Inputs, Level 2 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	1,247	3,496
Fair Value, Inputs, Level 2 | U.S. treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	1,000
Fair Value, Inputs, Level 2 | Debt securities of U.S. government-sponsored agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	512
Fair Value, Inputs, Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	0	0
Fair Value, Inputs, Level 3 | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	0	0
Fair Value, Inputs, Level 3 | Corporate debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	0	0
Fair Value, Inputs, Level 3 | Certificates of deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	0	0
Fair Value, Inputs, Level 3 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	0	$ 0
Fair Value, Inputs, Level 3 | U.S. treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	0
Fair Value, Inputs, Level 3 | Debt securities of U.S. government-sponsored agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets measured at fair value on a recurring basis	$ 0

Strategic Alliances and Colla43

Strategic Alliances and Collaborations - Revenue from Strategic Alliances (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenue from External Customer [Line Items]
Total revenues	$ 18	$ 204	$ 483	$ 489	$ 10,860	$ 1,865	$ 3,834	$ 4,200	$ 1,194	$ 20,759	$ 7,669
Strategic Alliances and Collaborations
Revenue from External Customer [Line Items]
Total revenues									1,194	20,759	7,669
Strategic Alliances and Collaborations | AstraZeneca
Revenue from External Customer [Line Items]
Total revenues									1,122	18,871	1,859
Strategic Alliances and Collaborations | Sanofi
Revenue from External Customer [Line Items]
Total revenues									72	71	979
Strategic Alliances and Collaborations | GSK
Revenue from External Customer [Line Items]
Total revenues									0	0	3,509
Strategic Alliances and Collaborations | Biogen
Revenue from External Customer [Line Items]
Total revenues									0	1,817	1,122
Strategic Alliances and Collaborations | Other
Revenue from External Customer [Line Items]
Total revenues									$ 0	$ 0	$ 200

Strategic Alliances and Colla44

Strategic Alliances and Collaborations (Details)

1 Months Ended

6 Months Ended

8 Months Ended

12 Months Ended

Dec. 31, 2015USD ($)shares

Sep. 30, 2015USD ($)

Jul. 31, 2015

May 31, 2015USD ($)

Mar. 31, 2015USD ($)

Jan. 31, 2015USD ($)

Aug. 31, 2014USD ($)

Feb. 28, 2014USD ($)$ / sharesshares

Jul. 31, 2013USD ($)

Oct. 31, 2012USD ($)$ / sharesshares

Aug. 31, 2012agreement

Jun. 30, 2015

Jan. 31, 2014USD ($)

Dec. 31, 2016USD ($)shares

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Common stock value

$ 53,000

$ 53,000

Common stock, shares issued | shares

52,669,266

52,924,805

AstraZeneca

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Number of collaborative areas granted | agreement

3

Initial upfront option payment

$ 3,000,000

Expected term of research and development plan

4 years

Common stock, shares issued | shares

6,250,000

Price per share (in dollars per share) | $ / shares

$ 4

Restriction period in which Alliances could not sell, transfer, make any short sale of, or grant any option for the sale of any common stock

365 days

Upfront payment received

$ 4,300,000

Expected revenue through milestone payments

$ 160,000,000

AstraZeneca | Preclinical

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payments, earned

2,500,000

AstraZeneca | Clinical

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payments, earned

$ 10,000,000

Expected revenue through milestone payments

32,500,000

AstraZeneca | Commercialization

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Expected revenue through milestone payments

$ 127,500,000

AstraZeneca | Side Letter

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Revenue recognized

$ 4,500,000

AstraZeneca | Side Letter | Preinvestigational New Drug And Active Pharmaceutical Ingredient Manufacturing Activities

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Percentage of cost funded

50.00%

AstraZeneca | Side Letter | Phase I clinical study product manufacturing activities

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Percentage of cost funded

100.00%

Sanofi

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Upfront payment received

$ 400,000

Sanofi | Private Placement

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Common stock value

$ 10,000,000

Common stock, shares issued | shares

1,303,780

Price per share (in dollars per share) | $ / shares

$ 7.67

Restriction period in which Alliances could not sell, transfer, make any short sale of, or grant any option for the sale of any common stock

12 months

Sanofi | Development Commercialization And License Agreement

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Initial upfront option payment

$ 2,500,000

$ 2,500,000

Upfront payment creditable against future accounts receivables

$ 1,250,000

$ 1,250,000

Upfront payment non-creditable portion recognized

$ 1,250,000

Deferred revenue creditable against future milestones

$ 1,250,000

Sanofi | Minimum | United States

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Royalties based on percentage of net sales

10.00%

Sanofi | Minimum | Outside of the US

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Royalties based on percentage of net sales

10.00%

Sanofi | Maximum | United States

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Royalties based on percentage of net sales

20.00%

Sanofi | Maximum | Outside of the US

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Royalties based on percentage of net sales

20.00%

Sanofi | Clinical

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Expected revenue through milestone payments

$ 15,000,000

Sanofi | Common Stock Purchase Agreement

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Deferred revenue

$ 200,000

Deferred revenue remaining recognition period (less than)

3 years

Sanofi | Proof-of-Concept Trial

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Expected revenue through milestone payments

$ 101,800,000

Sanofi | Regulatory and Commercialization Milestones

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Expected revenue through milestone payments

$ 300,000,000

Biogen

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Initial upfront option payment

$ 2,000,000

Deferred revenue

$ 0

Deferred revenue remaining recognition period (less than)

14 months

12 months

Revenue recognition period

1 year

Biogen | Research and development

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payments, earned

$ 300,000

$ 300,000

$ 100,000

Revenue recognized

$ 2,000,000

Property and Equipment, net (De

Property and Equipment, net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 19,633	$ 11,850
Less accumulated depreciation and amortization	(7,803)	(6,450)
Property and equipment, net	11,830	5,400
Depreciation and amortization expense	2,276	1,591	$ 1,490
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	7,932	7,310
Computer equipment and software
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	265	305
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	706	138
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	10,469	1,930
Construction in progress
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 261	$ 2,167

Intangible Assets (Details)

Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	$ 1,481	$ 1,460
Accumulated amortization	(466)	(379)
Intangibles, net	1,015	1,081
Intangible asset amortization	100	100	$ 100
Finite-lived intangible assets, amortization expense, next 12 months	100
Finite-lived intangible assets, amortization expense, year two	100
Finite-lived intangible assets, amortization expense, year three	100
Finite-lived intangible assets, amortization expense, year four	100
Finite-lived intangible assets, amortization expense, year five	100
Patents
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	1,102	1,081
Licenses
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	$ 379	$ 379

Commitments and Contingencies47

Commitments and Contingencies (Details) - USD ($)	1 Months Ended		12 Months Ended
	Jul. 31, 2015	Jul. 31, 2010	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Aug. 31, 2015
Other Commitments [Line Items]
Term of lease	96 months
Tenant incentive	$ 8,500,000					$ 1,400,000
Restricted cash			$ 0	$ 1,256,000		$ 1,400,000
Tenant improvement allowance used			8,200,000
Unused tenant imporvement allowance cap			400,000
Unused tenant improvement			300,000
Minimum period for early termination, in months		60 months
Minimum notice for early termination, in months		9 months
Future sublease income			200,000
Rent expense			1,200,000	600,000	$ 700,000
Deferred rent, less current portion			8,840,000	2,308,000
Prior Lease
Other Commitments [Line Items]
Other liabilities, current			200,000
Other Noncurrent Liabilities
Other Commitments [Line Items]
Deferred rent, less current portion			10,100,000	0
Other Noncurrent Liabilities | Prior Lease
Other Commitments [Line Items]
Deferred rent, less current portion			$ 0	$ 400,000

Commitments and Contingencies -

Commitments and Contingencies - Summary of Future Annual Minimum Lease Payments for Operating Leases (Details) $ in Thousands	Dec. 31, 2016USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Operating Leases, Future Minimum Payments Due in 2017	$ 2,330
Operating Leases, Future Minimum Payments, Due in 2018	2,576
Operating Leases, Future Minimum Payments, Due in 2019	2,654
Operating Leases, Future Minimum Payments, Due in 2020	2,733
Operating Leases, Future Minimum Payments, Due in 2021	2,815
Operating Leases, Future Minimum Payments, Due Thereafter	6,892
Operating Leases, Future Minimum Payments Due	$ 20,000

Convertible Note Payable - Addi

Convertible Note Payable - Additional Information (Details) - USD ($)	Jan. 29, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Oct. 31, 2012
Debt Instrument [Line Items]
Number of shares resulting from conversion of note payable	1,356,738
Conversion price per share (dollars per share)	$ 4
Loss from valuation of convertible note payable		$ 0	$ (1,811,000)	$ (12,118,000)
Convertible Debt | GSK
Debt Instrument [Line Items]
Notes payable					$ 5,400,000

Term Loan (Details)

Term Loan (Details)	Jun. 22, 2016USD ($)	Jun. 17, 2016USD ($)loans	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Jun. 30, 2016USD ($)
Debt Instrument [Line Items]
Proceeds from borrowing under term loan			$ 19,768,000	$ 0	$ 0
Term loan, less debt issuance costs			$ 19,802,000	$ 0
Debt Financing Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 30,000,000
Number of term loans | loans		2
Debt instrument, face amount						$ 20,000,000
Period to borrow additional funds - achievement of a defined milestone event until the earlier		60 days
Period for principal and interest payments		24 months
Stated interest rate, percentage		8.51%
Prepayment fee, option one			2.00%
Prepayment fee, option two			1.00%
Debt extinguishment fee			5.50%
Term loan, less debt issuance costs			$ 20,000,000
Amounts borrowed			20,000,000
Deferred finance costs			$ 200,000			$ 200,000
Debt instrument effective rate			8.98%
Debt Financing Agreement | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate, percentage		0.44%
Debt Financing Agreement Tranche A
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 20,000,000
Proceeds from borrowing under term loan	$ 20,000,000
Term loan, less debt issuance costs			$ 20,000,000
Debt Financing Agreement Tranche B
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 10,000,000

Term Loan - Future Principal Pa

Term Loan - Future Principal Payments (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Long-term Debt	$ 19,802	$ 0
Debt Financing Agreement Tranche A
Debt Instrument [Line Items]
Future Repayments of Principal, 2017	0
Future Repayments of Principal, 2018	5,000
Future Repayments of Principal, 2019	10,000
Future Repayments of Principal, 2020	5,000
Long-term Debt	$ 20,000

Common Stock and Stockholders52

Common Stock and Stockholders' Equity (Narrative) (Details)	Dec. 31, 2016voteshares	Dec. 31, 2015shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares outstanding (in shares)	52,924,805	52,669,266
Common stock voting rights per share (vote) | vote	1
Common stock options outstanding (in shares)	8,931,498	5,126,000
Total common shares reserved for future issuance (in shares)	11,244,249
Common stock available for future grant under the 2012 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock available for future grant under the 2012 Plan (in shares)	718,299
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock available for future grant under the 2012 Plan (in shares)	1,594,452
Common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares outstanding (in shares)	52,924,805

Common Stock and Stockholders53

Common Stock and Stockholders' Equity - Stock Option Activity (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016USD ($)$ / sharesshares
Number of options
Beginning number of options (in shares), outstanding | shares	5,126,000
Number of options, Granted (in shares) | shares	4,966,000
Number of options, Exercised (in shares) | shares	(90,000)
Number of options, Canceled/forfeited/expired (in shares) | shares	(1,071,000)
End number of options (in shares), outstanding | shares	8,931,498
Number of options, Vested or expected to vest (in shares) | shares	8,278,000
Number of options, Exercisable (in shares) | shares	2,882,000
Weighted average exercise price
Beginning, Weighted average exercise price, Options outstanding (in dollars per share) | $ / shares	$ 8.91
Weighted average exercise price, Granted (in dollars per share) | $ / shares	4.93
Weighted average exercise price, Exercised (in dollars per share) | $ / shares	3.45
Weighted average exercise price, Canceled/forfeited/expired (in dollars per share) | $ / shares	9.32
Ending, Weighted average exercise price, Options outstanding (in dollars per share) | $ / shares	6.70
Weighted average exercise price, Vested or expected to vest (in dollars per share) | $ / shares	6.82
Weighted average exercise price, Exercisable (in dollars per share) | $ / shares	$ 7.58
Weighted average remaining contractual term
Weighted average remaining contractual term, Options outstanding	8 years 1 month 21 days
Weighted average remaining contractual term, Vested or expected to vest	8 years 15 days
Weighted average remaining contractual term, Exercisable	5 years 11 months 27 days
Aggregate intrinsic value
Aggregate intrinsic value, Options outstanding | $	$ 34,680
Aggregate intrinsic value, Vested or expected to vest | $	31,463
Aggregate intrinsic value, Exercisable | $	$ 9,688

Common Stock and Stockholders54

Common Stock and Stockholders' Equity - Assumptions Used to Estimate Fair Value of Stock Options and Performance Stock and Employee Stock Purchase Plan (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Stock options
Weighted average assumptions
Risk-free interest rate	1.50%	1.80%	1.80%
Volatility	82.70%	78.90%	74.30%
Dividend yield	0.00%	0.00%	0.00%
Expected term (years)	6 years	6 years 1 month 6 days	6 years 1 month 6 days
Performance stock options
Weighted average assumptions
Risk-free interest rate	1.40%	1.80%	2.10%
Volatility	82.40%	76.70%	69.60%
Dividend yield	0.00%	0.00%	0.00%
Expected term (years)	5 years 10 months 24 days	6 years	6 years 3 months 18 days
Employee Stock Purchase Plan
Weighted average assumptions
Risk-free interest rate	0.50%	0.10%	0.10%
Volatility	93.70%	76.10%	69.80%
Dividend yield	0.00%	0.00%	0.00%
Expected term (years)	6 months	6 months	6 months
Minimum
Weighted average assumptions
Forfeiture rate	3.00%	3.00%
Maximum
Weighted average assumptions
Forfeiture rate	7.00%	7.00%

Common Stock and Stockholders55

Common Stock and Stockholders' Equity - Schedule of Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expenses	$ 12,872	$ 15,368	$ 7,039
Research and development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expenses	5,458	8,075	3,939
General and administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expenses	$ 7,414	$ 7,293	$ 3,100

Common Stock and Stockholders56

Common Stock and Stockholders' Equity (Narrative 2) (Details) - USD ($) $ / shares in Units, $ in Thousands	Jul. 17, 2015	Oct. 31, 2012	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Weighted average grant date fair value of employee stock options (in dollars per share)			$ 3.47	$ 7.47	$ 8.37
Total intrinsic value of stock options exercised			$ 200	$ 21,400	$ 12,100
Proceeds from stock options exercised			310	$ 6,680	$ 2,233
Compensation cost related to non-vested awards not yet recognized			$ 12,300
Weighted-average recognition period of compensation cost related to non-vested awards not yet recognized			1 year 6 months
Employee Stock Purchase Plan 2012
Class of Stock [Line Items]
Percentage of participants eligible compensation		15.00%
Common stock, shares issued under purchase plan			165,684
Shares issued (in shares)			321,184
Common stock shares reserved for future issuance and authorized for purchase			1,594,452
Option One - at inception of enrollment period
Class of Stock [Line Items]
Percentage of fair market value of common stock		85.00%
Option Two - at applicable purchase date
Class of Stock [Line Items]
Percentage of fair market value of common stock		85.00%
Inducement Plan
Class of Stock [Line Items]
Common stock shares reserved for future issuance and authorized for purchase	1,000,000
Vesting period	4 years
Awards vesting, first anniversary	25.00%
Remaining vesting period after initial 25%, on a monthly basis (months)	36 months
Maximum | Inducement Plan
Class of Stock [Line Items]
Award expiration period	10 years

Defined Contribution Plan (Deta

Defined Contribution Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Compensation and Retirement Disclosure [Abstract]
Defined contribution minimum age requirement of eligible employees	18 years
Maximum 401 (K) plan contribution rates as percentage of employees compensation	50.00%
Rate of contribution to 401(K) plan as a percentage of employees contribution	25.00%
Percentage of employee's earnings the company may contribute to the 401(k) plan	6.00%
Employer contribution to 401(k) plan	$ 0.2	$ 0.1	$ 0.1

Income Taxes - Components of In

Income Taxes - Components of Income Tax (Benefit) Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current:
Federal	$ 0	$ 0	$ 0
State	1	1	1
Current Income Tax Expense (Benefit), total	1	1	1
Deferred:
Federal	(5)	(17)	0
State	0	(2)	0
Deferred Income Tax Expense (Benefit), total	(5)	(19)	0
Income tax (benefit) expense	$ (4)	$ (18)	$ 1

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Expected Statutory Federal Income Tax Provision to Actual Income Tax Provision (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Expected income tax benefit at federal statutory tax rate	$ (27,826)	$ (18,960)	$ (19,271)
State income taxes, net of federal benefit	(1,152)	(1,580)	(2,348)
Tax credits	(5,447)	(5,039)	(3,307)
Change in fair value of convertible note payable	0	616	4,120
Change in valuation allowance	31,990	23,216	20,047
Prior year true-up	(1,320)	110	(92)
Stock compensation	2,458	1,161	902
Reserve for uncertain tax positions	1,314	254	0
Other	(21)	204	(50)
Income tax (benefit) expense	$ (4)	$ (18)	$ 1

Income Taxes - Components of De

Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Net operating loss carryovers	$ 70,739	$ 48,912
Research and development and other tax credits	22,613	15,148
Deferred revenue	735	1,210
Intangibles and property and equipment basis difference	0	2,097
Stock compensation expense	5,664	3,488
Other	4,679	1,129
Total deferred tax assets	104,430	71,984
Total deferred tax liabilities	(799)	(343)
Net deferred tax asset	103,631	71,641
Valuation allowance	(103,631)	(71,641)
Net deferred tax asset	$ 0	$ 0

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating Loss Carryforwards [Line Items]
Net deferred tax assets	$ 103,631,000	$ 71,641,000
Net operating loss carryovers	70,739,000	48,912,000
Federal Alternative Minimum Tax Credit carryforwards	$ 200,000
Net operating loss carryforward limitation on use percentage ownership change minimum	50.00%
Unrecognized tax benefits	$ 5,632,000	2,298,000	$ 1,853,000	$ 1,092,000
Unrecognized tax benefits that would not impact effective tax rate	3,700,000
Tax penalties and interest	0	$ 0	$ 0
Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Net operating loss carryovers	204,800,000
Internal Revenue Service (IRS) | Research Tax Credit Carryforward
Operating Loss Carryforwards [Line Items]
Research and development tax credits carryforward	20,300,000
Internal Revenue Service (IRS) | Stock options
Operating Loss Carryforwards [Line Items]
Net operating loss carryovers	19,400,000
California
Operating Loss Carryforwards [Line Items]
Net operating loss carryovers	173,200,000
California | Research Tax Credit Carryforward
Operating Loss Carryforwards [Line Items]
Research and development tax credits carryforward	6,900,000
California | Stock options
Operating Loss Carryforwards [Line Items]
Net operating loss carryovers	$ 19,400,000

Income Taxes - Changes in Amoun

Income Taxes - Changes in Amount of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance of unrecognized tax benefits	$ 2,298	$ 1,853	$ 1,092
Increase (decrease) for prior year tax positions	1,991	(250)	(73)
Increase for current year tax positions	1,343	695	834
Total	$ 5,632	$ 2,298	$ 1,853

Related Party Transactions (Det

Related Party Transactions (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Sanofi
Related Party Transaction [Line Items]
Expenses incurred for services performed or out-of-pocket expenses	$ 900,000	$ 400,000	$ 0
Alnylam
Related Party Transaction [Line Items]
Potential milestone payments to related party	$ 33,000,000

Selected Quarterly Financial 64

Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 18	$ 204	$ 483	$ 489	$ 10,860	$ 1,865	$ 3,834	$ 4,200	$ 1,194	$ 20,759	$ 7,669
Operating expenses	19,762	19,396	21,671	21,867	18,221	15,210	25,015	17,071	82,696	75,517	52,579
Net loss	$ (20,020)	$ (19,519)	$ (21,090)	$ (21,207)	$ (7,226)	$ (13,000)	$ (21,035)	$ (14,487)	$ (81,836)	$ (55,748)	$ (56,680)
Basic and diluted net loss per share (dollars per share)	$ (0.38)	$ (0.37)	$ (0.40)	$ (0.40)					$ (1.55)	$ (1.08)	$ (1.29)
Basic net loss per share (dollars per share)					$ (0.14)	$ (0.25)	$ (0.41)	$ (0.29)