8. Warrant Liabilities

In April 2011, the Company issued warrants to purchase 24,235 shares of Series A-1 convertible preferred stock, or Series A-1 warrants, and 83,771 warrants to purchase shares of Series B convertible preferred stock, or Series B warrants. The Series A-1 warrants and Series B warrants were immediately exercisable and expire, if not exercised, in April 2021 and April 2016, respectively. As the shares into which the warrants were exercisable were contingently redeemable, the Company recognized a liability for the fair value of the warrants on the condensed consolidated balance sheet.

At the date of the IPO, the Series A-1 warrants and Series B warrants became exercisable for common stock and were no longer contingently redeemable. At the IPO, the ending fair value of these warrants of $1.2 million was reclassified to additional paid-in capital, and the change in fair value of $1.1 million was recognized as loss from remeasurement of fair value of warrants in the condensed consolidated statements of operations.

In April, June, and October 2011, the Company issued warrants to purchase 615,658 shares of common stock. The common stock warrants were exercisable beginning in April 2015 and would have terminated in whole or part, if the Company had obtained certain levels of government grant funds by April 15, 2015. The warrants expire, if not exercised, in April 2021. As the warrants were subject to performance conditions which may result in the issuance of a variable number of shares, the Company recognized a liability for the fair value of the common stock warrants on the condensed consolidated balance sheet.

At April 15, 2015, the Company did not obtain the specified levels of government grant funds and the performance conditions expired and the number of common shares issuable was fixed. On April 15, 2015, the ending fair value of the common stock warrants of $25.9 million was reclassified to additional paid-in capital, and the change in fair value of $25.0 million was recognized as loss from remeasurement of fair value of warrants in the condensed consolidated statements of operations.

The key assumptions used in the Black-Scholes option-pricing model for the valuation of the convertible preferred stock warrants were as follows:

	Nine Months Ended September 30,
	2015	2014
Expected term (in years)	1.00 - 6.04	1.79 - 7.04
Fair value of underlying shares	$10.80 - $12.24	$0.67 - $1.56
Volatility	79.2% -111.1%	63.1% - 80.8%
Risk-free interest rate	0.23% - 1.54%	0.38% - 2.30%
Dividend yield	—%	—%

The key assumptions used in the Black-Scholes option-pricing model for the valuation of the common stock warrants were as follows:

	Nine Months Ended September 30,
	2015	2014
Expected term (in years)	6.00 - 6.58	6.79 - 7.58
Fair value of underlying shares	$15.00 - $42.00	$1.03 - $1.07
Volatility	82.0% - 82.6%	76.3% - 80.4%
Risk-free interest rate	1.51% - 1.63%	2.13% - 2.41%
Dividend yield	—%	—%