The following table summarizes stock option activity for the three months ended March 31, 2022 (in thousands):

	Number of Underlying Shares
Outstanding at December 31, 2021		2,288
Granted		—
Exercised		(116	)
Forfeited or expired		(5	)
Outstanding at March 31, 2022		2,167
Vested and exercisable at March 31, 2022		1,603

We grant (i) RSUs with a service condition, (ii) RSUs with performance and service conditions (“PSUs”), and (iii) RSUs with market and service conditions (“MSUs”) as further explained below.

We historically paid annual bonuses to our senior executives and other bonus-eligible employees with PSUs. In fiscal year 2022, we will pay employees’ bonus half with cash and half with PSUs. The number of annual PSUs that will ultimately vest depends on us attaining financial metrics for the year as well as on an employee’s continued employment through the vesting date. The compensation committee and board of directors typically certify PSU attainment in the first quarter of each year.

On April 12, 2021 and May 20, 2021, we granted a total of 83,750 MSUs to certain executives. The MSUs are eligible to vest based on our total stockholder return (“TSR”) relative to the TSR of the constituents in the S&P Semiconductor Select Industry Index over two measurement periods. Half of the MSUs are eligible to vest based on our relative TSR from January 1, 2021 through December 31, 2022, and half of the MSUs are eligible to vest based on our relative TSR from January 1, 2021 through December 31, 2023. We use a Monte Carlo simulation in estimating the fair value at grant date and recognize compensation cost over the implied service period. We estimated the aggregate grant-date fair value of these shares to be $6.4 million using the Monte Carlo simulation valuation method.

On March 23, 2022 we granted a total of 57,000 MSUs to certain executives. The MSUs are eligible to vest based on our TSR relative to the TSR of the constituents in the S&P Semiconductor Select Industry Index over two measurement periods. Half of the MSUs are eligible to vest based on our relative TSR from January 1, 2022 through December 31, 2023, and half of the MSUs are eligible to vest based on our relative TSR from January 1, 2022 through December 31, 2024. We use a Monte Carlo simulation in estimating the fair value at grant date and recognize compensation cost over the implied service period. We estimated the aggregate grant-date fair value of these shares to be $4.6 million using the Monte Carlo simulation valuation method.

The following table summarizes activity for RSUs, PSUs, and MSUs, for the three months ended March 31, 2022 (in thousands):

	Number of Underlying Shares
	RSUs			MSUs		PSUs
Outstanding at December 31, 2021		1,165			84		268
Granted		591			57		75
Vested		(59	)		—		(268	)
Forfeited		(12	)		—		—
Outstanding at March 31, 2022		1,685			141		75

We recorded $749,000 of stock-based compensation expense related to the MSUs for the three months ended March 31, 2022.

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,
	2022		2021
Cost of revenue	$	599	$	289
Research and development expense		4,775		3,110
Sales and marketing expense		2,752		1,802
General and administrative expense		3,188		2,248
Total stock-based compensation expense	$	11,314	$	7,449