Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2017

Fair Value Measurements.

Assumptions used to calculate common stock warrants

	Three months ended
	March 31, 2017	March 31, 2016
Risk-free interest rate	1.01% - 2.01%	0.30%  - 1.04%
Volatility	62.0% - 105.20%	56.47% - 108.92%
Expected average term	0.89 - 5.23	0.17 - 2.79

Schedule of activity in the common stock warrant liability

The following table shows the activity in the common stock warrant liability (in thousands):

	Three months ended
Common stock warrant liability	March 31, 2017		March 31, 2016
Beginning of period	$	11,387	$	5,735
Change in fair value of common stock warrants		2,280		(1,278)
Exercise of common stock warrants		—		(140)
End of period	$	13,667	$	4,317