Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2018

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

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



	Three Months Ended March 31,
	2018		2017
Research and development	$	1,700	$	1,673
General and administrative	1,732		1,788
Total stock-based compensation expense	$	3,432	$	3,461

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,
	2018	2017
Expected dividend yield	0%	0%
Expected volatility	68% - 71%	67%
Risk-free interest rate	2.4% - 2.8%	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 three months ended March 31, 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	1,038,874		28.49
Exercised	(176,616	)	5.03
Forfeited or expired	(51,423	)	20.90
Outstanding, March 31, 2018	5,315,477		21.97		7.4	$	29,749
As of March 31, 2018:
Exercisable	2,940,789		18.61		6.1	25,431
Vested and expected to vest	5,074,332		21.76		7.4	29,334