The following table summarizes stock-based compensation expense by financial statement line item in the Company’s condensed consolidated statements of operations for each of the three months ended June 30, 2022 and 2021:


	For the Three Months Ended June 30,
	2022		2021
	(in thousands)
Cost of revenue	$	1,396	$	1,030
Research and development		2,798		2,109
Selling, general and administrative		9,168		9,469
	$	13,362	$	12,608

The following table summarizes the stock option activity for the three months ended June 30, 2022:



						Weighted
				Weighted		Average		Aggregate
				Average		Remaining		Intrinsic
	Options			Exercise		Contractual		Value
	(in thousands)			Price		Term (years)		(in thousands)
Outstanding at beginning of period		595		$	178.54		5.55
Granted		2			275.58
Exercised		(18	)		84.37
Cancelled and expired		(1	)		284.46
Outstanding at end of period		578		$	181.61		5.37	$	50,807
Exercisable at end of period		503		$	168.81		4.89	$	50,099
Options vested and expected to vest at end of period		578		$	181.61		5.37	$	50,807

Stock options generally vest and become exercisable annually over three years. The remaining unrecognized stock-based compensation expense for unvested stock option awards as of June 30, 2022, was approximately $6.5 million and the weighted-average period over which this cost is expected to be recognized is 1.8 years.

The aggregate intrinsic value of stock options exercised was $3.7 million for the three months ended June 30, 2022. The total cash received as a result of employee stock option exercises for the three months ended June 30, 2022, was approximately $1.5 million.

The Company estimates the fair value of each stock option granted at the grant date using the Black-Scholes option valuation model. The weighted average grant-date fair values and weighted average assumptions used in the calculation of fair value of options granted were as follows:


	For the Three Months Ended June 30,
	2022			2021
Weighted average grant-date fair value	$	121.69		$	103.03

Valuation assumptions:
Risk-free interest rate		3.01	%		0.79	%
Expected option life (years)		5.54			4.20
Expected volatility		43.51	%		44.28	%

The following table summarizes activity of restricted stock units for the three months ended June 30, 2022:


				Weighted
				Average
	Number of			Grant Date
	Shares			Fair Value
	(in thousands)			(per share)
Restricted stock units at beginning of period		307		$	274.32
Granted (1)		258			266.07
Vested		(106	)		282.80
Forfeited		(3	)		272.26
Restricted stock units at end of period		456		$	267.00

(1) Includes 19,000 performance-based awards granted due to greater than 100% target vesting.

The weighted average grant-date fair value for restricted stock units granted during the three months ended June 30, 2022 was $266.1. The total fair value of restricted stock units vested during the three months ended June 30, 2022 was $29.9 million.

Restricted stock units generally vest annually, over three years. The remaining unrecognized compensation expense for outstanding restricted stock units, including performance-based and market-based awards, as of June 30, 2022 was $100.1 million and the estimated weighted-average period over which this cost is expected to be recognized is 2.4 years.

As of June 30, 2022, the Company recognized compensation expense based on the probable outcomes related to the prescribed performance targets on the outstanding awards. The remaining unrecognized compensation expense for outstanding performance-based and market-based restricted stock units as of June 30, 2022 was $33.7 million and the weighted-average period over which this cost is expected to be recognized is 2.2 years.

The Company grants performance-based restricted stock units to certain executive officers and employees, which vest upon achievement of prescribed service-based milestones by the award recipients and the achievement of prescribed performance milestones by the Company, as defined in the respective agreements.

The Company grants market-based restricted stock units to certain executive officers and employees. These restricted stock units vest upon achievement of prescribed service-based milestones, relative total shareholder return (“TSR”) goals by the Company and the achievement of prescribed performance milestones by the Company, as defined in the respective agreements.

The Company used a Monte-Carlo simulation model to estimate the grant-date fair value of the TSR restricted stock units. The fair value related to these awards is recorded as compensation expense over the vesting term, regardless of the actual TSR outcome reached.

The table below sets forth the assumptions used to value the outstanding market-based restricted stock units and the estimated grant-date fair value:


	May 2021			May 2020
Risk-free interest rate		0.3	%		0.2	%
Expected volatility		44.8	%		35.5	%
Dividend yield		—			—
Remaining performance period (years)	2.8				2.9
Estimated fair value per share	$	292.40		$	349.28
Target performance (number of shares)		25,172			15,425