STOCK-BASED COMPENSATION (Tables)

3 Months Ended

Mar. 31, 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	116,050		$	7.50
Exercised	(15,000	)	$	2.80
Canceled	(53,739	)	$	17.28
Options Outstanding at March 31, 2016	8,157,550		$	13.04	6.5
Options Vested and Expected to Vest at March 31, 2016	8,110,040		$	13.00	6.5
Options Exercisable at March 31, 2016	4,948,179		$	8.82	5.2
Shares Available for Grant under the 2008 Plan	5,727,572

Schedule of stock-based compensation expense

	Three months ended March 31,
	2016		2015
	(In thousands)
Research and development	$	1,815	$	1,269
General and administrative	2,125		1,012
Total stock-based compensation expense	$	3,940	$	2,281

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

	Three months ended March 31,
	2016		2015
Expected stock price volatility	70	%	69	%
Expected option term	6.0 Years		6.0 Years
Risk-free interest rate	1.6	%	1.8	%
Expected dividend yield	None		None