The following table summarizes stock option activity for the three months ended March 31, 2021 (in thousands):

	Number of Underlying Shares
Outstanding at December 31, 2020		3,061
Granted		6
Exercised		(332	)
Forfeited or expired		(12	)
Outstanding at March 31, 2021		2,723
Vested and exercisable at March 31, 2021		1,357

The following table summarizes activity for restricted stock units, or RSUs, and RSUs with performance conditions, or PSUs, for the three months ended March 31, 2021 (in thousands):

	Number of Underlying Shares
	RSUs			PSUs
Outstanding at December 31, 2020		836			251
Granted		4			—
Vested		(54	)		(241	)
Forfeited		(7	)		(10	)
Outstanding at March 31, 2021		779			—

We began granting PSUs in 2019 under our annual bonus program to our senior executives and other bonus-eligible employees. The number of annual PSUs that ultimately vest depends on us attaining financial metrics for the fiscal year as well as on the employee’s continued employment through the vesting date. The compensation committee and board of directors certified achievement of the financial metric for PSUs granted in 2020, vesting 241,000 shares in first-quarter 2021.

The following table presents stock-based compensation expense included in our condensed consolidated statements of operations for the periods presented (in thousands):

	Three Months Ended March 31,
	2021		2020
Cost of revenue	$	289	$	207
Research and development expense		3,110		2,021
Sales and marketing expense		1,802		1,368
General and administrative expense		2,248		1,625
Total stock-based compensation expense	$	7,449	$	5,221