Stock-Based Compensation

6 Months Ended

Jun. 30, 2015

Stock-Based Compensation

Stock-Based Compensation

(10)  Stock-Based Compensation

A summary of stock option activity for the six months ended June 30, 2015 is as follows:

	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	1,734,650		$	25.41
Exercised	(574,554	)	$	5.55
Canceled	(54,263	)	$	14.41
Options Outstanding at June 30, 2015	8,121,183		$	13.00	7.1
Options Vested and Expected to Vest at June 30, 2015	8,059,516		$	12.94	7.1
Options Exercisable at June 30, 2015	4,247,785		$	7.95	5.4
Shares Available for Grant under the 2008 Plan	5,905,772

The weighted average grant-date fair value of stock options granted during the six months ended June 30, 2015 was $15.63. Stock-based compensation expense for the three and six months ended June 30, 2015 and 2014 was recorded as follows:

	Three months ended June 30,				Six months ended June 30,
	2015		2014		2015		2014
	(In thousands)
Research and development	$	1,244	$	747	$	2,512	$	1,338
General and administrative	1,283		764		2,296		1,423
Total stock-based compensation expense	$	2,527	$	1,511	$	4,808	$	2,761

The fair values of employee and director stock options granted during the three and six months ended June 30, 2015 and 2014 were valued using the Black-Scholes option-pricing model with the following assumptions:

	Three months ended June 30,				Six months ended June 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	2.2	%	2.2	%	1.8 - 2.2	%	2.2	%
Expected dividend yield	None		None		None		None