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Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stock-Based Compensation Expense

The following table shows stock-based compensation expense for stock options, RSUs and ESPP (in thousands):

			Three Months Ended September 30,																		Nine Months Ended September 30,
			2018						2017						2018						2017
Research and development			$	2,440					$	2,006					$	6,347					$	5,512
General and administrative			2,133						1,995						5,821						5,552
Total stock-based compensation expense			$	4,573					$	4,001					$	12,168					$	11,064

Valuation Assumptions Using the Black-Scholes Option-Pricing Model

The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions in the following table for options issued during the period indicated:

			Nine Months Ended September 30,
			2018						2017
Expected dividend yield			—%						—%
Expected volatility			67.8% - 72.2%						66.7% - 68.3%
Risk-free interest rate			2.4% - 3.1%						1.9% - 2.3%
Expected term			6.25 years						6.25 years

Stock Option and Restricted Stock Unit Activity

The following table summarizes stock option and restricted stock unit (RSU) activity during the nine months ended September 30, 2018:

			Shares						Weighted- Average Exercise Price						Weighted-Average Remaining Contractual Term (Years)						Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2017			4,504,642						$	19.79					7.0
Granted			1,230,244						27.43
Exercised			(205,902)						5.47
Forfeited or expired			(180,709)						24.25
Outstanding, September 30, 2018			5,348,275						21.95						7.0						$	19,762
As of September 30, 2018:
Exercisable			3,365,982						19.70						6.0						18,892
Vested and expected to vest			5,138,801						21.79						7.0						19,671

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Tabular disclosure of the allocation of equity-based compensation costs to a given line item on the balance sheet and income statement for the period. This may include the reporting line for the costs and the amount capitalized and expensed.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions.

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