The following table summarizes stock-based compensation expense included in operating expenses (in thousands):


	For the Three Months Ended June 30,				For the Six Months Ended June 30,
	2023		2022		2023		2022
Cost of sales	$	104	$	55	$	185	$	110
Research and development		1,902		6,953		3,840		9,921
Selling, general and administrative		4,054		8,085		7,424		12,398
Total	$	6,060	$	15,093	$	11,449	$	22,429

In 2023, we began granting performance-based restricted stock units (“PSUs”) to certain employees, which will vest only if challenging performance goals relating to revenue and relative total stockholder return are achieved over a three-year performance period and challenging performance goals relating to clinical milestones are achieved over a four year performance period.

Stock-based compensation expense for all PSUs is determined using the grant date fair value, which is the quoted closing market price per share of our common stock on the Nasdaq Global Select Market on the grant date, except that the grant date fair value of PSUs related to relative total stockholder return includes a discount which was calculated using a Monte Carlo simulation and represents the likelihood that the related performance goals will not be achieved.

Stock-based compensation expense for PSUs related to relative total stockholder return is recognized on a straight-line basis over the service period. Stock-based compensation expense for all other PSUs is not recognized until the achievement of the performance goal is deemed probable (the “Probable Date”), a determination that requires significant judgment by management, as the achievement of these goals have inherent risk and uncertainties. At the Probable Date, we record a cumulative catch-up expense for the portion of the grant date fair value attributable to the period from the grant date to the Probable Date. The remaining expense is recognized over the remaining service period on a straight-line basis.

On May 19, 2022, our stockholders approved the 2022 Equity Incentive Plan (the “2022 Plan”). The 2022 Plan provides for the grant of incentive stock options, non-statutory stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards and other stock-based awards. On May 24, 2023, our stockholders approved an amendment to the 2022 Plan to increase the number of shares of our common stock available for issuance under the 2022 Plan by 5,000,000 shares.

On May 24, 2023, our stockholders approved an amendment and restatement (the “Amended and Restated ESPP”) of our 2013 Employee Stock Purchase Plan (the “ESPP”). The Amended and Restated ESPP (1) eliminated the evergreen provision in the ESPP, which historically provided for an automatic annual increase in the ESPP’s share reserve and (2) increased the number of shares of common stock authorized for issuance under the ESPP from 2,508,923 shares to 4,008,923 shares.