STOCK COMPENSATION (Tables)

6 Months Ended

Jun. 30, 2013

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock Option Activity

A summary of the Company’s stock option activity with respect to the six months ended June 30, 2013 follows:

Stock Options	Underlying Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
Outstanding at December 31, 2012		2,522,522		$	11.76
Granted		1,754,170			34.10
Exercised		(119,547	)		8.35
Canceled		(180,956	)		9.60
Outstanding at June 30, 2013		3,976,189		$	21.82		8.91	$	64,737,000
Vested at June 30, 2013 and expected to vest		3,639,329		$	21.27		8.84	$	61,269,000
Exercisable at June 30, 2013		714,247		$	11.81		6.82	$	18,963,000

Assumptions for Measuring Fair Values of Stock Options Granted

The fair values of stock options granted during the period presented were measured on the date of grant using the Black-Scholes-Merton option-pricing model, with the following assumptions:

	Three and Six Months Ended June 30,
	2013			2012
Risk-free interest rate		0.7% - 1.4	%		0.8% - 1.1	%
Expected dividend yield		0	%		0	%
Expected lives		5.0 years			5.3 years
Expected volatility		80.0% - 84.1.	%		79.7% - 82.5	%

(1) For the three and six months ended June 30, 2013, expected volatility has been estimated using a blend of calculated volatility of the Company’s common stock over a historical period and implied volatility in exchange-traded options associated with the Company’s common stock. Prior to January 1, 2013, expected volatility has been estimated using calculated volatility of the Company’s common stock over a historical period commensurate with the expected term of the option.

Restricted Stock Unit Activity

For the six months ended June 30, 2013, restricted stock unit activity is summarized in the following table:

	Six Months Ended June 30,
	2013
	Shares		Weighted Average Grant Date Fair Value per Share
Restricted Stock Units, beginning of period		38,260	$	6.32
Granted		—		—
Vested		24,594		6.83
Canceled		341		5.40
Restricted Stock Units, end of period		13,325	$	5.40

Summary of Restricted Stock Activity

The following table sets forth restricted stock activity for the period shown:

	Six Months Ended June 30, 2013
	Shares		Weighted Average Grant Date Fair Value per Share
Restricted Stock Awards, beginning of period		4,998	$	10.08
Granted		6,000		34.92
Vested		—		—
Canceled		—		—
Restricted Stock Awards, end of period		10,998		23.63

Summary of Stock-Based Compensation Expense Recognized in Statements of Operations

A summary of the stock-based compensation expense, including stock options, restricted stock, RSUs, and SARs recognized in the condensed consolidated statements of operations and comprehensive loss is as follows:

	Three Months Ended				Six Months Ended
	June 30, 2013		June 30, 2012		June 30, 2013		June 30, 2012
	(in thousands)				(in thousands)
Research and development	$	724	$	259	$	1,254	$	512
General and administrative		1,594		181		2,735		636
Total	$	2,318	$	440	$	3,989	$	1,148