The following table presents all stock-based compensation as included on the condensed consolidated statements of operations (in thousands):

	Three Months Ended June 30,					Six Months Ended June 30,
	2020			2019		2020		2019
	(Unaudited)					(Unaudited)
Stock-based compensation expense:
Employee stock options	$	192		$	104	$	297	$	1,101
Employee RSUs		83			313		373		563
Non-employee RSUs		41			105		91		209
	$	316		$	522	$	761	$	1,873
Stock-based compensation expense in operating expenses:
Research and development	$	(55	)	$	150	$	73	$	261
Selling, general and administrative		371			372		688		1,612
	$	316		$	522	$	761	$	1,873

The following table summarizes stock option activity under all equity incentive plans for the six months ended June 30, 2020:

	Number of Shares			Weighted- Average Exercise Price		Aggregate Intrinsic Value (in thousands)		Weighted- Average Remaining Contractual Life (in years)
Outstanding at December 31, 2019		4,506,950		$	9.37	$	5,710		5.8
Options granted		400,000		$	6.21
Options exercised		(259,017	)	$	2.18
Options forfeited		(116,667	)	$	3.07
Outstanding at June 30, 2020		4,531,266		$	9.67	$	30,354		5.7
Vested and Exercisable at June 30, 2020		3,806,264		$	10.59	$	24,933		5.0

The total unrecognized compensation cost related to non-vested stock options as of June 30, 2020 is $2.4 million, which is expected to be recognized over a weighted-average period of 1.3 years.

During the three and six months ended June 30, 2020, we recognized proceeds of $0.6 million and $0.6 million, respectively, from exercises of stock options. The aggregate intrinsic value of stock options exercised during the three and six months ended June 30, 2020, was $1.9 million and $1.9 million, respectively. During the three and six months ended June 30, 2019, we recognized proceeds of $0 and $4.1 million, respectively, from exercises of stock options by our Chairman and CEO.

The company uses a Black-Scholes option-pricing model to determine the fair value of stock-based compensation under U.S. GAAP. The assumptions used for employee stock options granted during the periods presented in these condensed consolidated financial statements are presented in the table below for the three and six months ended June 30, 2020 (Unaudited):

The expected term was estimated using the average of the contractual term and the weighted-average vesting term of the options. The risk-free interest rate assumption was based on the U.S. Treasury’s rates for U.S. Treasury zero-coupon bonds with maturities similar to those of the expected term of the award being valued. Expected volatility was estimated based on the historical volatility of our common stock. The assumed dividend yield was based on the company’s expectation of not paying dividends for the foreseeable future.

The following table summarizes the restricted stock units, or RSUs, activity under the 2015 Plan for the six months ended June 30, 2020:

	Number of Shares			Weighted- Average Grant Date Fair Value
Unvested balance at December 31, 2019		1,139,428		$	2.23
Granted		33,500		$	6.43
Vested		(558,976	)	$	1.79
Forfeited		(48,150	)	$	4.86
Unvested balance at June 30, 2020		565,802		$	2.70

As of June 30, 2020, there was $0.9 million of unrecognized stock-based compensation expense related to RSUs that is expected to be recognized over a weighted-average period of 2.1 years. Of that amount, $0.9 million of unrecognized expense is related to employee grants with a remaining weighted-average period of 2.2 years and $37,100 of unrecognized expense is related to non-employee grants with a remaining weighted-average period of 0.5 years.

During the three and six months ended June 30, 2019, we recognized proceeds of $0 and $35.2 million, respectively, upon the exercise of warrants by our Chairman and CEO. As of December 31, 2019 there were no warrants outstanding, and no warrants were issued during the six months ended June 30, 2020.