The following table summarizes stock option activity for the six months ended June 30, 2021 (in thousands):

	Number of Underlying Shares
Outstanding at December 31, 2020		3,061
Granted		10
Exercised		(384	)
Forfeited or expired		(42	)
Outstanding at June 30, 2021		2,645
Vested and exercisable at June 30, 2021		1,572

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.

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 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 shares of MSUs, to certain executives. The MSUs are eligible to vest based on our total stockholder return (“TSR”) relative to the TSR of the constituents comprising the S&P Semiconductor Select Industry Index over two measurement periods. Half of the MSUs are eligible to vest based on our relative TSR during the period from January 1, 2021 through December 31, 2022, and half of the MSUs are eligible to vest based on our relative TSR during the period 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. The aggregate grant-date fair value of these shares was estimated to be $6.4 million using the Monte Carlo simulation valuation method.

The following table summarizes activity for RSUs, PSUs, and MSUs, for the six months ended June 30, 2021 (in thousands):

	Number of Underlying Shares
	RSUs			MSUs		PSUs
Outstanding at December 31, 2020		836			—		251
Granted		621			84		265
Vested		(203	)		—		(241	)
Forfeited		(21	)		—		(10	)
Outstanding at June 30, 2021		1,233			84		265

We recorded $558,000 of stock-based compensation expense related to the MSUs for the three and six months ended June 30, 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 June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Cost of revenue	$	471	$	176	$	760	$	383
Research and development expense		4,568		1,640		7,678		3,661
Sales and marketing expense		2,320		1,204		4,122		2,572
General and administrative expense		3,223		1,577		5,471		3,202
Total stock-based compensation expense	$	10,582	$	4,597	$	18,031	$	9,818