STOCK-BASED COMPENSATION (Tables)

12 Months Ended

Dec. 31, 2013

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, 2012		5,349,810		$	5.98		7.0
Granted		1,252,000		$	16.44
Exercised		(816,405	)	$	6.53
Canceled		(14,861	)	$	6.97
Options Outstanding at December 31, 2013		5,770,544		$	8.17		7.0
Options Vested and Expected to Vest at December 31, 2013		5,712,596		$	8.12		7.0
Options Exercisable at December 31, 2013		3,297,190		$	6.30		5.6
Shares Available for Grant under the 2008 Plan		2,102,942

Summary of restricted stock activity

	Shares			Weighted Average Grant Date Fair Value (per share)
Outstanding and unvested at December 31, 2012		6,000		$	4.50
Granted		12,000		$	15.20
Vested		(12,000	)	$	9.85
Canceled		—			—
Outstanding and unvested at December 31, 2013		6,000		$	15.20

Schedule of stock-based compensation expense

	2013		2012		2011
	(In thousands)
Research and development	$	2,640	$	1,415	$	1,412
General and administrative		1,908		883		930
Total stock-based compensation expense	$	4,548	$	2,298	$	2,342

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

	Year Ended December 31, 2013	Year Ended December 31, 2012	Year Ended December 31, 2011
Expected stock price volatility	71 - 72%	70 - 72%	68 - 70%
Expected option term	6.0 Years	6.0 Years	6.0 Years
Risk-free interest rate	1.2 - 2.1%	0.9 - 1.4%	1.4 - 2.9%
Expected dividend yield	None	None	None