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Stock-Based Compensation (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Stock-based compensation expense
Stock-based compensation expense	$ 1,683	$ 801	$ 4,490	$ 2,359
Research and development
Stock-based compensation expense
Stock-based compensation expense	902	612	2,363	1,797
General and administrative
Stock-based compensation expense
Stock-based compensation expense	781	189	2,127	562
Employee stock options
Stock-based compensation expense
Stock-based compensation expense	1,620	765	4,276	2,222
Restricted stock
Stock-based compensation expense
Stock-based compensation expense	$ 63	$ 36	$ 214	$ 137

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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