Stock-Based Compensation

6 Months Ended

Jun. 30, 2014

Stock-Based Compensation

Stock-Based Compensation

(10)  Stock-Based Compensation

A summary of stock option activity for the six months ended June 30, 2014 is as follows:

	Shares		Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term (In Years)
Options Outstanding at December 31, 2013	5,770,544		$	8.17	7.0
Granted	1,333,850		$	13.57
Exercised	(134,131	)	$	6.05
Canceled	(8,497	)	$	10.51
Options Outstanding at June 30, 2014	6,961,766		$	9.24	7.2
Options Vested and Expected to Vest at June 30, 2014	6,886,817		$	9.19	7.2
Options Exercisable at June 30, 2014	3,480,200		$	6.18	5.3
Shares Available for Grant under the 2008 Plan	765,589

The weighted average grant-date fair value of stock options granted during the six months ended June 30, 2014 was $8.76. Stock-based compensation expense for the three and six months ended June 30, 2014 and 2013 was recorded as follows:

	Three months ended June 30,				Six months ended June 30,
	2014		2013		2014		2013
	(In thousands)
Research and development	$	747	$	422	$	1,338	$	863
General and administrative	764		254		1,423		521
Total stock-based compensation expense	$	1,511	$	676	$	2,761	$	1,384

The fair values of employee and director stock options granted during the three and six months ended June 30, 2014 and 2013 were valued using the Black-Scholes option-pricing model with the following assumptions:

	Three months ended June 30,				Six months ended June 30,
	2014		2013		2014		2013
Expected stock price volatility	72	%	72	%	71 - 72	%	72	%
Expected option term	6.0 years		6.0 years		6.0 years		6.0 years
Risk-free interest rate	2.2	%	1.2 - 1.8	%	2.2	%	1.2 - 1.8	%
Expected dividend yield	None		None		None		None