Stock Compensation Plans (Tables)

12 Months Ended

Dec. 31, 2017

The following table summarizes stock option activity under the 2012 and 2014 Plans:



	Options		Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)	Aggregate Intrinsic Value (In thousands)
Outstanding as of January 1, 2017	4,841,274		$	7.78
Granted	1,405,054		$	3.28
Exercised	(2,889	)	$	1.84
Forfeited and expired	(159,957	)	$	7.38
Outstanding as of December 31, 2017	6,083,482		$	6.76	7.0	$	6,709
Options vested and expected to vest as of December 31, 2017	5,980,525		$	6.79	6.9	$	6,530
Options exercisable as of December 31, 2017	3,562,490		$	7.94	5.7	$	2,491

The following table summarizes information about stock options (in thousands):



	Year Ended December 31,
	2017		2016		2015
Aggregate intrinsic value of options exercised	$	10	$	39	$	1,235

The following are the ranges of underlying assumptions used to determine the fair value of stock options granted to employees and non-employees:



	Years Ended December 31,
	2017		2016		2015
Employees and Directors:
Risk-free interest rate	1.5% - 2.0%		1.5% - 1.7%		1.7% - 1.9%
Expected dividend yield	—	%	—	%	—	%
Expected volatility	82% - 85%		77% - 86%		73% - 92%
Expected term of options (years)	5.9 - 6.1		5.9 - 6.0		5.8 - 6.0
Range of common stock value	$2.75 - $5.80		$5.90 - $8.97		$4.57 - $26.71



Non-Employees:
Risk-free interest rate	1.0% - 2.1%		0.5% - 1.9%		0.1% - 1.9%
Expected dividend yield	—	%	—	%	—	%
Expected volatility	66% - 84%		77% - 97%		56% - 94%
Expected term of options (years)	0.5 - 4.5		0.2 - 5.5		0.3 - 6.0
Range of common stock value	$2.90 - $5.95		$4.35 - $9.07		$4.04 - $25.01

The following tables summarize the allocation of stock-based compensation expense to employees and non-employees (in thousands):