STOCK-BASED COMPENSATION (Tables)

6 Months Ended

Jun. 30, 2014

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for each of the periods indicated:

	For the Three Months Ended June 30,										For the Six Months Ended June 30,
	2014					2013					2014					2013
	Shares			Weighted Average Exercise Price		Shares			Weighted Average Exercise Price		Shares			Weighted Average Exercise Price		Shares			Weighted Average Exercise Price
Grants outstanding at beginning of the period		5,273,338		$	24.27		2,694,678		$	13.35		4,190,367		$	23.46		2,522,522		$	11.76
Granted		117,770			33.75		1,511,850			34.95		1,303,035			27.45		1,754,170			34.10
Exercised		(26,025	)		14.64		(55,157	)		7.63		(74,604	)		11.87		(119,547	)		8.35
Canceled or expired		(136,710	)		23.11		(175,182	)		9.43		(190,425	)		25.13		(180,956	)		9.60
Grants outstanding at end of the period		5,228,373			24.56		3,976,189		$	21.82		5,228,373			24.56		3,976,189		$	21.82
Grants exercisable at end of the period		1,642,252			17.75		714,247		$	11.81		1,642,252			17.75		714,247		$	11.81

Summary of Restricted Stock Award Activity

The following table summarizes the Company’s RSA activity for each of the periods indicated:

	For the Three Months Ended June 30,									For the Six Months Ended June 30,
	2014					2013				2014					2013
	Shares			Weighted Average Grant Date Fair Value		Shares		Weighted Average Grant Date Fair Value		Shares			Weighted Average Grant Date Fair Value		Shares		Weighted Average Grant Date Fair Value
Grants outstanding at beginning of the period		12,000		$	31.98		4,998	$	10.08		6,000		$	34.92		4,998	$	10.08
Granted		—			—		6,000		34.92		6,000			29.03		6,000		34.92
Vested		(6,000	)		34.92		—		—		(6,000	)	$	34.92		—		—
Grants outstanding at end of the period		6,000		$	29.03		10,998		23.63		6,000			29.03		10,998	$	23.63

Summary of Restricted Stock Unit Activity

The following table summarizes the Company’s RSU activity for each of the periods indicated:

	For the Three Months Ended June 30,										For the Six Months Ended June 30,
	2014					2013					2014					2013
	Shares			Weighted Average Grant Date Fair Value		Shares			Weighted Average Grant Date Fair Value		Shares			Weighted Average Grant Date Fair Value		Shares			Weighted Average Grant Date Fair Value
Grants outstanding at beginning of the period		6,507		$	5.40		37,998		$	6.33		6,507		$	5.40		38,260		$	6.32
Vested		(6,507	)		5.40		(24,594	)		6.83		(6,507	)		5.40		(24,594	)		6.83
Canceled or expired		—			—		(79	)		7.16		—			—		(341	)		5.40
Grants outstanding at end of the period		0			0		13,325		$	5.40		0			0		13,325		$	5.40

Summary of Stock Appreciation Rights Activity

The following table summarizes the Company’s SAR activity for each of the periods indicated:

	For the Three and Six Months Ended June 30,
	2014				2013
	Shares		Weighted Average Exercise Price		Shares		Weighted Average Grant Date Fair Value
Grants outstanding at beginning of the period		170,000	$	18.18		170,000	$	18.18
Grants outstanding at end of the period		170,000	$	18.18		170,000	$	18.18
Grants exercisable at end of the period		71,666	$	17.69		—	$	18.18

Summary of Stock-Based Compensation Expense by Function Included within Condensed Consolidated Statement of Operations and Comprehensive Loss

The following table summarizes stock-based compensation expense by function included within the unaudited condensed consolidated statements of operations and comprehensive loss:

	For the Three Months Ended June 30,				For the Six Months Ended June 30,
	2014		2013		2014		2013
	(in thousands)
Research and development	$	2,345	$	724	$	4,218	$	1,254
General and administrative		3,242		1,594		5,711		2,735
Total	$	5,587	$	2,318	$	9,929	$	3,989

Summary of Stock-Based Compensation Expense by Grant Type Included within Condensed Consolidated Statement of Operations and Comprehensive Loss

The following table summarizes stock-based compensation expense by grant type included within the unaudited condensed consolidated statements of operations and comprehensive loss:

	For the Three Months Ended June 30,				For the Six Months Ended June 30,
	2014		2013		2014		2013
	(in thousands)
Stock options	$	5,042	$	2,032	$	8,953	$	3,433
Restricted stock awards		72		27		136		39
Restricted stock units		5		111		1		228
Stock appreciation rights		146		148		293		289
Employee stock purchase plan		322		—		546		—
Total	$	5,587	$	2,318	$	9,929	$	3,989

Stock Options [Member]

Assumptions for Measuring Fair Values of Stocks

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:

	For the Three Months Ended June 30,		For the Six Months Ended June 30,
	2014	2013	2014	2013
Risk-free interest rate (1)	1.5 – 1.7%	0.7 – 1.4%	1.5 – 1.7%	0.7 – 1.4%
Expected dividend yield (2)	0%	0%	0%	0%
Expected lives (3)	4.9 years	5.0 years	4.9 years	5.0 years
Expected volatility (4)	93.0%	80.0 – 84.1%	93.0 – 95.4%	80.0 – 84.1%

(1) The risk-free interest rate is estimated using an average of Treasury bill interest rates over a historical period commensurate with the expected term of the option that correlates to the prevailing interest rates at the time of grant.

(2) The expected dividend yield is zero as the Company has not paid any dividends to date and does not expect to pay dividends in the future.

(3) The expected lives are estimated using expected and historical exercise behavior.

(4) The expected volatility is estimated using a blend of calculated volatility of the Company’s common stock over a historical period and implied volatility in exchange-traded options of the Company’s common stock.