6. Warrant Liabilities

Liabilities measured at fair value on a recurring basis include warrant liabilities resulting from our equity financings. In accordance with ASC 815-40, Derivatives and Hedging – Contracts in Entity's Own Equity ("ASC 815-40"), the warrant liabilities are recorded at fair value until they are completely settled. The warrants are valued using the Black-Scholes method, using assumptions consistent with the Company's application of ASC 505-50, Equity-Based Payments to Non-Employees ("ASC 505-50"). The gain or loss resulting from the change in fair value is shown on the Condensed Statements of Operations as gain (loss) on warrant derivative liability. We recognized a loss of $4.3 million and a gain of $0.9 million for the three-month periods ended June 30, 2017 and 2016, respectively, and a loss of $4.3 million and a gain of $0.7 million for the six-month periods ended June 30, 2017 and 2016, respectively. The following reflects the weighted-average assumptions for each of the six-month periods indicated:

	Six Months Ended June 30,
	2017			2016

Risk-free interest rate		1.13	%		0.20	%
Expected dividend yield		0	%		0	%
Expected lives		0.79			0.09
Expected volatility		125.3	%		59.3	%
Warrants classified as liabilities (in shares)		24,110,577			6,371,854

Our computation of expected volatility is based on the historical daily volatility of our publicly traded stock. The dividend yield assumption of zero is based upon the fact that we have never paid cash dividends and presently have no intention to do so. The risk-free interest rate used for each warrant classified as a derivative is equal to the U.S. Treasury rates in effect at June 30 of each year presented. The expected lives are based on the remaining contractual lives of the related warrants at the valuation date.

On July 19, 2017, 6,340,148 warrants expired.