6. Warrant Liabilities

Liabilities measured at market value on a recurring basis include warrant liabilities resulting from the Company's past equity financings, including the underwritten public offering that closed on August 1, 2011. In accordance with ASC 815-40, Derivatives and Hedging – Contracts in Entity's Own Equity ("ASC 815-40"), the warrant liabilities are being marked to market until they are completely settled. The warrants are valued using the Black-Scholes method, using assumptions consistent with our application of ASC 505-50, Equity-Based Payments to Non-Employees ("ASC 505-50"). The gain or loss resulting from the marked to market calculation is shown on the Consolidated Statements of Operations as gain (loss) on warrant derivative liability. The Company recognized a loss on warrant derivative liability of $2.1 million and $3.9 million for the three-month periods ended March 31, 2013 and 2012, respectively. The following reflects the weighted-average assumptions for each of the three-month periods indicated:


	Three MonthsEnded March 31,
		2013			2012

Risk-free interest rate			0.45	%		0.74	%
Expected dividend yield			0	%		0	%
Expected lives			3.21			4.15
Expected volatility			69.0	%		89.2	%
Warrants classified as liabilities		$	6,067,450		$	3,972,230
Loss on warrant liabilities		$	(2,095,220	)	$	(3,888,166	)

The dividend yield assumption of zero is based upon the fact that the Company has never paid cash dividends and presently has no intention of paying cash dividends. The risk-free interest rate used for each grant is equal to the U.S. Treasury rates in effect at the time of the grant for instruments with a similar expected life. The expected lives are based on the remaining contractual lives of the related warrants at the valuation date.