Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2013

Summary of common shares that were excluded from the diluted net loss per share calculation because of their anti-dilutive effect

The following table summarizes potential common shares that were excluded from diluted net loss per share calculation because of their anti-dilutive effect:



	Year Ended December 31,
	2013	2012	2011
Options to purchase common stock	16,538,118	16,257,444	14,302,803
Warrants to purchase common stock	14,059,260	21,593,844	21,743,844
Convertible preferred stock	38,233	38,233	38,233
Total	30,635,611	37,889,521	36,084,880

Summary of assumptions used to estimate the fair value of stock options

Weighted average assumptions used in the Black-Scholes model for employees and directors are presented below:



	Year Ended December 31,
	2013	2012	2011
Risk-free interest rate	0.41%	0.36%	1.23%
Expected volatility	111%	131%	134%
Expected life in years	4	4	4
Dividend yield	—	—	—
Forfeiture rate	8%	8%	11%

Assumptions used in the Black-Scholes model for non-employees are presented below:



	Year Ended December 31,
	2013	2012	2011
Risk-free interest rate	0.84% – 1.52%	0.67% – 0.83%	0.91% – 3.47%
Expected volatility	102%-113%	103%-117%	96%-119%
Expected life in years	7-10	7-10	6-10
Dividend yield	—	—	—