Fair Value of Financial Instruments (Tables)

3 Months Ended

Mar. 31, 2013

Fair Value of Financial Instruments [Abstract]

Assets and liabilities measured at fair value

The table below categorizes assets and liabilities measured at fair value on a recurring basis as of March 31, 2013 and December 31, 2012:

	Fair Value		Fair value measurement using
	March 31, 2013		Level 1		Level 2		Level 3
Assets:
Money Market	$	22,877	$	22,877	$	–	$	–
Certificate of Deposit		400		400		–		–
Total Assets	$	23,277	$	23,277	$	–	$	–
Liabilities:
Common stock warrant liability	$	6,143	$	–	$	–	$	6,143

	Fair Value		Fair value measurement using
	December 31, 2012		Level 1		Level 2		Level 3
Assets:
Money Market	$	23,377	$	23,377	$	–	$	–
Certificate of Deposit		400		400		–		–
Total Assets	$	23,777	$	23,777	$	–	$	–
Liabilities:
Common stock warrant liability	$	6,305	$	–	$	–	$	6,305

Common stock warrants measured at Level 3 inputs on recurring basis

The table below summarizes the activity of Level 3 inputs measured on a recurring basis for the three months ended March 31, 2013 and 2012:

(in thousands)	Fair Value Measurements of Common Stock Warrants Using Significant Unobservable Inputs (Level 3)
Balance at December 31, 2012	$	6,305
Change in fair value of common stock warrant liability		(162	)
Balance at March 31, 2013	$	6,143

(in thousands)	Fair Value Measurements of Common Stock Warrants Using Significant Unobservable Inputs (Level 3)
Balance at December 31, 2011	$	6,996
Exercise of warrants		(126	)
Change in fair value of common stock warrant liability		3,434
Balance at March 31, 2012	$	10,304

Significant unobservable input assumption used for valuation

The significant unobservable inputs used in the fair value measurement of common stock warrants are the historical volatility of our common stock market price, expected term of the applicable warrants, and the risk-free interest rate based on the U.S. Treasury yield curve in effect at the measurement date. In addition to the significant unobservable inputs noted above, the fair value measurement of certain five-year warrants issued in February 2011 also takes into account an assumption of the likelihood and timing of the occurrence of an event that would result in an adjustment to the exercise price in accordance with the anti-dilutive pricing provisions in the warrant. Any significant increases or decreases in the unobservable inputs, with the exception of the risk-free interest rate, would result in significantly higher or lower fair value measurements.

Significant Unobservable Input Assumptions of Level 3 Valuations	March 31, 2013			December 31, 2012
Historical Volatility		60%-79	%		56% -80	%
Expected Term (in years)		1.1 – 2.9			1.4 – 3.2
Risk-free interest rate		0.14% - 0.36	%		0.16% - 0.36	%