STOCK-BASED COMPENSATION (Tables)

12 Months Ended

Dec. 31, 2015

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, 2014		7,015,350		$	9.34		6.6
Granted		1,865,250		$	24.75
Exercised		(700,561	)	$	5.81
Canceled		(69,800	)	$	15.79
Options Outstanding at December 31, 2015		8,110,239		$	13.13		6.7
Options Vested and Expected to Vest at December 31, 2015		8,065,256		$	13.08		6.7
Options Exercisable at December 31, 2015		4,705,304		$	8.53		5.3
Shares Available for Grant under the 2008 Plan		5,790,709

Summary of restricted stock activity

	Shares			Weighted Average Grant Date Fair Value (per share)
Outstanding and unvested at December 31, 2014		7,000		$	14.78
Granted		39,000		$	25.41
Vested		(26,500	)	$	22.60
Canceled		—			—
Outstanding and unvested at December 31, 2015		19,500		$	25.41

Schedule of stock-based compensation expense

	2015		2014		2013
	(In thousands)
Research and development	$	6,186	$	3,459	$	2,640
General and administrative		6,588		3,393		1,908
Total stock-based compensation expense	$	12,774	$	6,852	$	4,548

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

	Year Ended December 31, 2015	Year Ended December 31, 2014	Year Ended December 31, 2013
Expected stock price volatility	67 - 69%	70 - 72%	71 - 72%
Expected option term	6.0 Years	6.0 Years	6.0 Years
Risk-free interest rate	1.8 - 2.2%	1.9 - 2.2%	1.2 - 2.1%
Expected dividend yield	None	None	None