The following is a summary of activity in the number of warrants outstanding to purchase the Company’s common stock for the three months ended March 31, 2014 (in thousands):

Warrants consist of warrants potentially settleable in cash, which are liability-classified warrants, and equity-classified warrants.

Warrants classified as liabilities

Liability-classified warrants consist of warrants issued in connection with equity financings in February 2012, August 2013, October 2013 and January 2014.  These warrants are potentially settleable in cash and were determined not to be indexed to the Company’s own stock and are therefore accounted for as liabilities.

The estimated fair value of outstanding warrants accounted for as liabilities is determined at each balance sheet date. Any decrease or increase in the estimated fair value of the warrant liability since the most recent balance sheet date is recorded in the consolidated statement of comprehensive loss as other income (expense). The fair value of the warrants classified as liabilities is estimated using the Black-Scholes option-pricing model with the following inputs as of March 31, 2014:

The Company’s expected volatility is based on a combination of implied volatilities of similar publicly traded entities given that the Company has limited history of its own observable stock price. The expected life assumption is based on the remaining contractual terms of the warrants. The risk-free rate is based on the zero coupon rates in effect at the balance sheet date. The dividend yield used in the pricing model is zero, because the Company has no present intention to pay cash dividends.

Warrants classified as equity

Warrants other than those in the above table were recorded in equity at fair value upon issuance, and are not reported as liabilities on the balance sheet.