STOCK-BASED COMPENSATION (Tables)

3 Months Ended

Mar. 31, 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 March 31,
	2014					2013
	Shares			Weighted Average Exercise Price		Shares			Weighted Average Exercise Price
Grants outstanding at beginning of the period		4,190,367		$	23.46		2,522,522		$	11.76
Granted		1,185,265			26.83		242,320			28.76
Exercised		(48,579	)		10.39		(64,390	)		8.97
Canceled or expired		(53,715	)		30.26		(5,774	)		14.83
Grants outstanding at end of the period		5,273,338		$	24.27		2,694,678		$	13.35
Grants exercisable at end of the period		1,180,245		$	12.85		656,805		$	12.36

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 March 31,
	2014				2013
	Shares		Weighted Average Grant Date Fair Value		Shares		Weighted Average Grant Date Fair Value
Grants outstanding at beginning of the period		6,000	$	34.92		4,998	$	10.08
Granted		6,000		29.03		—		—
Grants outstanding at end of the period		12,000	$	31.98		4,998	$	10.08

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 March 31,
	2014				2013
	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		38,260		$	6.32
Canceled or expired		—		—		(262	)		5.40
Grants outstanding at end of the period		6,507	$	5.40		37,998		$	6.33

Summary of Stock Appreciation Rights Activity

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

	For the Three Months Ended March 31,
	2014				2013
	Shares		Weighted Average Exercise Price		Shares		Weighted Average Exercise Price
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		61,041	$	17.60		—	$	—

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

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

	For the Three Months Ended March 31,
	2014		2013
Research and development	$	1,873	$	530
General and administrative		2,469		1,141
Total	$	4,342	$	1,671

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

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

	For the Three Months Ended March 31,
	2014			2013
Stock options	$	3,911		$	1,401
Restricted stock awards		64			12
Restricted stock units		(4	)		117
Stock appreciation rights		147			141
Employee stock purchase plan		224			—
Total	$	4,342		$	1,671

Stock Options

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 March 31,
	2014			2013
Risk-free interest rate (1)		1.7	%		0.7	%
Expected dividend yield (2)		0	%		0	%
Expected lives (3)		4.9 years			4.9 years
Expected volatility (4)		95.4	%		80.0	%

(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.

Employee Stock Purchase Plan

Assumptions for Measuring Fair Values of Stocks

The following table summarizes the assumptions used in the calculation during the period indicated:

	For the Three Months Ended March 31, 2014
Fair value of stock purchase rights	$12.22 - 25.35
Weighted-average stock price volatility	101% - 109%
Expected lives	0.5 - 2 years
Risk-free rate	0.1% - 0.3%