Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2016

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, 2015	8,110,239		$	13.13	6.7
Granted	2,291,250		$	4.86
Exercised	(37,942	)	$	2.93
Canceled	(103,474	)	$	15.92
Options Outstanding at June 30, 2016	10,260,073		$	11.29	7.0
Options Vested and Expected to Vest at June 30, 2016	10,175,654		$	11.30	7.0
Options Exercisable at June 30, 2016	5,558,678		$	10.20	5.3
Shares Available for Grant under the 2008 Plan	3,542,107

Schedule of stock-based compensation expense

	Three months ended June 30,				Six months ended June 30,
	2016		2015		2016		2015
	(In thousands)
Research and development	$	2,008	$	1,244	$	3,824	$	2,512
General and administrative	1,965		1,283		4,089		2,296
Total stock-based compensation expense	$	3,973	$	2,527	$	7,913	$	4,808

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

	Three months ended June 30,				Six months ended June 30,
	2016		2015		2016		2015
Expected stock price volatility	77	%	67	%	70 - 77	%	67 - 69	%
Expected option term	6.0 years		6.0 years		6.0 years		6.0 years
Risk-free interest rate	1.5	%	2.2	%	1.5 – 1.6	%	1.8 - 2.2	%
Expected dividend yield	None		None		None		None