Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2017

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

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



	Three Months Ended March 31,
	2017		2016
Research and development	$	1,673	$	1,396
General and administrative	1,788		1,605
Total stock-based compensation expense	$	3,461	$	3,001

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,
	2017	2016
Expected dividend yield	0%	0%
Expected volatility	67%	64%
Risk-free interest rate	2.3%	1.5% - 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 three months ended March 31, 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	1,031,068		20.43
Exercised	(109,826	)	0.98
Forfeited or expired	(15,784	)	26.97
Outstanding, March 31, 2017	4,743,518		19.64		7.6	$	19,885
As of March 31, 2017:
Exercisable	2,397,928		15.08		6.2	18,531
Vested and expected to vest	4,495,867		19.37		7.5	19,697