Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2017

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 September 30,				Nine Months Ended September 30,
	2017		2016		2017		2016
Research and development	$	2,006	$	1,403	$	5,512	$	4,243
General and administrative	1,995		1,600		5,552		4,883
Total stock-based compensation expense	$	4,001	$	3,003	$	11,064	$	9,126

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,
	2017	2016
Expected dividend yield	0%	0%
Expected volatility	66.7% - 68.3%	63.7% - 67.7%
Risk-free interest rate	1.9% - 2.3%	1.2% - 2.1%
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, 2017:



	Shares		Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)	Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2016	3,838,060		$	18.93	7.0
Granted	1,219,725		20.09
Exercised	(219,732	)	2.02
Forfeited or expired	(304,259	)	24.13
Outstanding, September 30, 2017	4,533,794		19.71		7.2	$	18,050
As of September 30, 2017:
Exercisable	2,704,057		16.86		6.2	17,254
Vested and expected to vest	4,352,976		19.48		7.2	17,993