Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2013

Classification of Stock-Based Compensation Expense

Stock-based compensation expense, net of estimated forfeitures, is reflected in the statements of operations as follows (in thousands):

	Year Ended December 31,
	2013		2012		2011
Research and development	$	2,295	$	1,452	$	1,164
General and administrative		2,679		1,357		1,189
Total stock-based compensation	$	4,974	$	2,809	$	2,353

Fair Values of Employee Stock Options

The estimated fair value of the above warrants was determined using the Black-Scholes option-pricing model using the following assumptions:

	May 22,		December 31,
	2013		2012
Risk-free interest rate	0.1 - 0.9%		0.3 - 0.6%
Estimated term equal to the remaining contractual term	1.7 - 3.8 years		2.1 - 4.2 years
Volatility		79%		82%
Dividend yield	–		–

Stock Option

Fair Values of Employee Stock Options

The following table illustrates the weighted-average assumptions for the Black-Scholes option-pricing model used in determining the fair value of options granted to employees:

	Year Ended December 31,
	2013	2012	2011
Risk-free interest rate	1.43%	1.10%	1.10%
Expected life	6.0 years	6.0 years	6.0 years
Expected volatility	79%	72%	85%
Dividend yield	–	–	–

Employee Stock Purchase Plan

Fair Values of Employee Stock Options

The following table illustrates the weighted-average assumptions for the Black-Scholes option-pricing model used in determining the fair value of ESPP purchase rights granted to employees:

	Year Ended December 31,
	2013	2012	2011
Risk-free interest rate	0.10%	–	–
Expected life	0.42 years	–	–
Expected volatility	62%	–	–
Dividend yield	–	–	–