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Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2019

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

Stock-Based Compensation Expense

The following stock-based compensation expense was recognized for the periods indicated (in thousands):

			Three Months Ended March 31,
			2019						2018
Research and development			$	1,814					$	1,700
General and administrative			1,985						1,732
Total stock-based compensation expense			$	3,799					$	3,432

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:

			Three Months Ended March 31,
			2019						2018
Expected dividend yield			—%						—%
Expected volatility			73.7% - 74.4%						68% - 71%
Risk-free interest rate			2.3% - 2.6%						2.4% - 2.8%
Expected term			6.25 years						6.25 years

Stock Option and Restricted Stock Unit Activity

The following table summarizes stock option activity during the three months ended March 31, 2019:

			Shares						Weighted- Average Exercise Price						Weighted-Average Remaining Contractual Term (Years)						Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2018			5,273,964						$	22.23					6.8
Granted			1,628,022						21.82
Exercised			(126,707)						2.84
Forfeited or expired			(55,032)						21.02
Outstanding, March 31, 2019			6,720,247						22.48						7.4						$	10,712
As of March 31, 2019:
Exercisable			3,593,089						21.44						5.9						10,670
Vested and expected to vest			6,407,781						22.41						7.4						10,707

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- Definition

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.

+ References

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

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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-Paragraph 1
-Subparagraph (b)
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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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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)
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