11. Stock-Based Compensation

The following table presents stock-based compensation expense included on the Company’s condensed consolidated statement of operations (in thousands):

	Three Months Ended March 31,
	(unaudited)
	2018		2017
Stock-based compensation expense:
Warrants for common stock to an officer	$	7,636	$	8,486
Employee stock options		978		1,351
Employee restricted stock units		392		318
Non-employee restricted stock units		67		(137	)
	$	9,073	$	10,018
Stock-based compensation expense in operating expenses:
Research and development	$	139	$	92
Selling, general and administrative		8,934		9,926
	$	9,073	$	10,018

Stock Options

The following table summarizes stock option activity for the three months ended March 31, 2018 (in thousands, except for share and year amounts):

	Number of Shares		Weighted- Average Exercise Price		Aggregate Intrinsic Value		Weighted- Average Remaining Contractual Life (in years)
Outstanding at December 31, 2017		5,693,250	$	7.71	$	11,920		5.3
Options granted/exercised/forfeited		—	$	-
Outstanding at March 31, 2018		5,693,250	$	7.71	$	8,987		5.0
Vested and Exercisable at March 31, 2018		5,230,375	$	8.20	$	8,192		5.4

The total unrecognized stock-based compensation expense related to non-vested stock options as of March 31, 2018 is $3.8 million, which is expected to be recognized over a weighted-average period of 1.0 years.

Restricted Stock Units

The following table summarizes the activity for restricted stock units:

	Number of Restricted Stock Units Outstanding			Weighted-Average Grant Date Fair Value
Unvested balance at December 31, 2017		888,189		$	8.14
Granted		139,300		$	3.74
Vested		(70,200	)	$	6.31
Forfeited		(83,125	)	$	8.13
Unvested balance at March 31, 2018		874,164		$	7.58

During the three months ended March 31, 2018, the Company granted 139,300 shares of restricted stock units to employees.

As of March 31, 2018, there was $3.7 million of unrecognized stock-based compensation expense related to restricted stock units that is expected to be recognized over a weighted-average period of 2.6 years. Of that amount, $3.1 million of unrecognized expense is related to employee grants with a weighted-average period of 2.9 years and $0.6 million of unrecognized expense is related to non-employee grants with a weighted-average period of 1.4 years that is impacted by periodic mark-to-market adjustments.

Warrants

The following table summarizes the warrant activity for the three months ended March 31, 2018:

Outstanding at December 31, 2017	17,721,088
Warrants exercised	(14,270	)
Outstanding at March 31, 2018	17,706,818
Vested and exercisable at March 31, 2018	16,966,218

The total unrecognized stock-based compensation expense related to non-vested warrants as of March 31, 2018 is $10.2 million, which is expected to be recognized over a weighted-average period of 0.3 years.