STOCK-BASED COMPENSATION (Tables)

12 Months Ended

Dec. 31, 2014

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, 2013		5,770,544		$	8.17		7.0
Granted		1,456,800		$	13.69
Exercised		(169,977	)	$	6.00
Canceled		(42,017	)	$	13.10
Options Outstanding at December 31, 2014		7,015,350		$	9.34		6.6
Options Vested and Expected to Vest at December 31, 2014		6,963,570		$	9.31		6.6
Options Exercisable at December 31, 2014		4,090,112		$	7.06		5.1
Shares Available for Grant under the 2008 Plan		675,159

Summary of restricted stock activity

	Shares			Weighted Average Grant Date Fair Value (per share)
Outstanding and unvested at December 31, 2013		6,000		$	15.20
Granted		14,000		$	14.12
Vested		(12,000	)	$	14.33
Canceled		(1,000	)	$	13.45
Outstanding and unvested at December 31, 2014		7,000		$	14.78

Schedule of stock-based compensation expense

	2014		2013		2012
	(In thousands)
Research and development	$	3,459	$	2,640	$	1,415
General and administrative		3,393		1,908		883
Total stock-based compensation expense	$	6,852	$	4,548	$	2,298

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

	Year Ended December 31, 2014	Year Ended December 31, 2013	Year Ended December 31, 2012
Expected stock price volatility	70 - 72%	71 - 72%	70 - 72%
Expected option term	6.0 Years	6.0 Years	6.0 Years
Risk-free interest rate	1.9 - 2.2%	1.2 - 2.1%	0.9 - 1.4%
Expected dividend yield	None	None	None