The following table summarizes stock-based compensation expense related to stock options, performance stock units (“PSUs”), restricted stock units (“RSUs”) and restricted stock for the three months ended March 31, 2020 and 2019:

	Three Months Ended
	March 31,
(In thousands)	2020			2019
Stock-based compensation expense included in cost of sales	$	115		$	104
Selling, general and administrative expense		1,075			1,063
Research and development expense		601			692
Stock-based compensation expense included in operating expenses		1,676			1,755
Total stock-based compensation expense		1,791			1,859
Tax benefit for expense associated with stock options, PSUs, RSUs and restricted stock		(427	)		(443	)
Total stock-based compensation expense, net of tax	$	1,364		$	1,416

The following table summarizes PSUs, RSUs and restricted stock outstanding as of December 31, 2019 and March 31, 2020 and the changes that occurred during the three months ended March 31, 2020.

	Number of Shares (in thousands)			Weighted Avg. Grant Date Fair Value (per share)
Unvested PSUs, RSUs and restricted stock outstanding, December 31, 2019		1,891		$	14.58
PSUs, RSUs and restricted stock granted		394		$	8.18
PSUs, RSUs and restricted stock vested		(13	)	$	12.32
PSUs, RSUs and restricted stock forfeited		(422	)	$	21.12
Unvested PSUs, RSUs and restricted stock outstanding, March 31, 2020		1,850		$	11.75

During the three months ended March 31, 2020, the Company issued 0.3 million performance-based PSUs to its executive officers and certain employees. The grant-date fair value of these performance-based awards is based on the closing price of the Company’s stock on the date of grant. These awards vest over a three-year period, subject to the grantee’s continued employment, with the ability to earn shares in a range of 0% to 142.8% of the awarded number of PSUs based on the achievement of defined performance targets. Equity-based compensation expense with respect to these awards will be adjusted over the vesting period to reflect the probability of achievement of performance targets defined in the award agreements.

The fair value of RSUs and restricted stock is equal to the closing price of our stock on the business day immediately preceding the grant date. The fair value of PSUs with market conditions is calculated using a Monte Carlo simulation valuation method.

As of March 31, 2020, total unrecognized compensation expense related to non-vested market-based PSUs, RSUs and restricted stock was approximately $15.3 million, which will be recognized over the remaining weighted-average period of 2.6 years. Unrecognized compensation expense will be adjusted for actual forfeitures.

As of March 31, 2020, 1.4 million shares were available for issuance under shareholder-approved equity plans in connection with the grant and exercise of stock options, PSUs, RSUs or restricted stock.

The following table summarizes stock options outstanding as of December 31, 2019 and March 31, 2020 and the changes that occurred during the three months ended March 31, 2020:

	Number of Stock Options (in thousands)			Weighted Avg. Exercise Price (per share)		Weighted Avg. Remaining Contractual Life (in years)		Aggregate Intrinsic Value (in thousands)
Stock options outstanding, December 31, 2019		3,572		$	22.88		3.4	$	—
Stock options exercised		—		$	—
Stock options forfeited		—		$	—
Stock options expired		(125	)	$	23.49
Stock options outstanding, March 31, 2020		3,447		$	22.86		3.1	$	—
Stock options exercisable, March 31, 2020		3,444		$	22.86		3.1	$	—

As of March 31, 2020, total unrecognized compensation expense related to non-vested stock options was approximately $7 thousand, which will be recognized over the remaining weighted-average period of 0.6 years. Unrecognized compensation expense will be adjusted for actual forfeitures.

There were no stock options granted during the three months ended March 31, 2020 and 2019. All of the options were previously issued at exercise prices that approximated fair market value at the date of grant.

The aggregate intrinsic value of stock options represents the total pre-tax intrinsic value (the difference between ADTRAN’s closing stock price on the last trading day of the quarter and the exercise price, multiplied by the number of in-the-money options) that would have been received by the option holders had all option holders exercised their options on March 31, 2020. The amount of aggregate intrinsic value will change based on the fair market value of ADTRAN’s stock and was zero as of March 31, 2020. The total pre-tax intrinsic value of options exercised during the three months ended March 31, 2020 was zero.