Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2015

Stock-Based Compensation

Summary of stock option activity

	Shares		Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term (In Years)
Options Outstanding at December 31, 2014	7,015,350		$	9.34	6.6
Granted	1,806,650		$	25.05
Exercised	(660,658	)	$	5.68
Canceled	(63,214	)	$	15.32
Options Outstanding at September 30, 2015	8,098,128		$	13.10	6.9
Options Vested and Expected to Vest at September 30, 2015	8,045,278		$	13.04	6.9
Options Exercisable at September 30, 2015	4,480,232		$	8.25	5.3
Shares Available for Grant under the 2008 Plan	5,842,723

Schedule of stock-based compensation expense

	Three Months Ended September 30,				Nine Months Ended September 30,
	2015		2014		2015		2014
	(In thousands)
Research and development	$	1,715	$	977	$	4,228	$	2,315
General and administrative	2,153		981		4,449		2,404
Total stock-based compensation expense	$	3,868	$	1,958	$	8,677	$	4,719

Schedule of assumptions used for the fair values of employee stock options granted

	Three Months Ended September 30,				Nine Months Ended September 30,
	2015		2014		2015		2014
Expected stock price volatility	67	%	72	%	67 - 69	%	71 - 72	%
Expected option term	6.0 years		6.0 years		6.0 years		6.0 years
Risk-free interest rate	1.9	%	2.1 - 2.2	%	1.8 - 2.2	%	2.1 - 2.2	%
Expected dividend yield	None		None		None		None