Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2017

Fair Value Measurements.

Assumptions used to calculate common stock warrants

	Six months ended
	June 30, 2017	June 30, 2016
Risk-free interest rate	1.01% - 2.36%	0.73%  - 0.86%
Volatility	62.00% - 108.77%	59.50% - 95.23%
Expected average term	0.64 - 10.00	1.64 - 2.55

Schedule of activity in the common stock warrant liability

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

	Six months ended
Common stock warrant liability	June 30, 2017		June 30, 2016
Beginning of period	$	11,387	$	5,735
Change in fair value of common stock warrants		14,576		(1,278)
Issuance of common stock warrants		4,905		—
Exercise of common stock warrants		(27,089)		(140)
End of period	$	3,779	$	4,317