STOCK-BASED COMPENSATION (Tables)

6 Months Ended

Jun. 30, 2013

STOCK-BASED COMPENSATION

Allocation of stock-based compensation expense included in the consolidated statements of operations

	Three Months Ended June 30,				Six Months Ended June 30,
(in thousands)	2013		2012		2013		2012
Research and development	$	4,508	$	3,541	$	8,305	$	7,070
General and administrative	2,654		2,438		4,952		5,144
Total stock-based compensation expense	$	7,162	$	5,979	$	13,257	$	12,214

Schedule of weighted-average assumptions used to estimate the fair value of stock options granted and employee stock purchase plan issuances

	Three Months Ended June 30,						Six Months Ended June 30,
	2013			2012			2013			2012
Employee stock options
Risk-free interest rate	0.76%-1.30		%	0.74%-1.10		%	0.76%-1.30		%	0.74%-1.17		%
Expected term (in years)	5-6			5-6			5-6			5-6
Volatility	59%-60		%	55%-60		%	58%-60		%	55%-60		%
Dividend yield	—		%	—		%	—		%	—		%
Weighted-average estimated fair value of stock options granted	$	17.64		$	11.11		$	15.43		$	10.37

The range of weighted-average assumptions used to estimate the fair value of employee stock purchase plan awards was as follows:

	Three Months Ended June 30,						Six Months Ended June 30,
	2013			2012			2013			2012
Employee stock purchase plan issuances
Risk-free interest rate	0.09%-0.26		%	0.15%-0.29		%	0.09%-0.26		%	0.15%-0.29		%
Expected term (in years)	0.5-2.0			0.5-2.0			0.5-2.0			0.5-2.0
Volatility	56%-61		%	55%-64		%	56%-61		%	55%-64		%
Dividend yield	—		%	—		%	—		%	—		%
Weighted-average estimated fair value of ESPP issuances	$	16.44		$	9.00		$	16.44		$	9.00