STOCK COMPENSATION (Tables)

9 Months Ended

Sep. 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 nine months ended September 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		2,063,396			34.86
Exercised		(196,012	)		8.51
Canceled		(239,072	)		11.51
Outstanding at September 30, 2013		4,150,834		$	23.37		8.71	$	99,056,000
Vested at September 30, 2013 and expected to vest		3,395,249		$	21.29		8.53	$	88,063,000
Exercisable at September 30, 2013		891,157		$	11.60		6.95	$	31,748,000

Assumptions for Measuring Fair Values of Stock Options Granted

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

	Three and Nine months ended September 30,
	2013	2012
Risk-free interest rate	0.7% - 1.4%	0.6% - 1.1%
Expected dividend yield	0%	0%
Expected lives	4.9-5.0 years	5.1-5.3 years
Expected volatility	80.0% - 88.9.%	79.7% - 94.8%

(1) For the three and nine months ended September 30, 2013, expected volatility was 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 was estimated using calculated volatility of the Company’s common stock over a historical period commensurate with the expected term of the option.

Summary of Restricted Stock Activity

The following table sets forth RSA activity for the period shown:

	Nine months ended September 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		(4,998	)		10.08
Restricted Stock Awards, end of period		6,000			34.92

Restricted Stock Unit Activity

The following table sets forth RSU activity for the period shown:

	Nine months ended September 30,
	2013
	Shares			Weighted Average Grant Date Fair Value per Share
Restricted Stock Units, beginning of period		38,260		$	6.32
Granted		—			—
Vested		(24,858	)		6.81
Canceled		(374	)		5.40
Restricted Stock Units, end of period		13,028		$	5.40

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

A summary of the stock-based compensation expense, including stock options, RSAs, RSUs, SARs, and ESPP rights recognized in the condensed consolidated statements of operations and comprehensive income (loss) is as follows:

	Three Months Ended				Nine months Ended
	September 30, 2013		September 30, 2012		September 30, 2013		September 30, 2012
	(in thousands)				(in thousands)
Research and development	$	1,155	$	271	$	2,409	$	783
General and administrative		2,332		421		5,067		1,057
Total	$	3,487	$	692	$	7,476	$	1,840