The following table summarizes stock-based compensation expense by financial statement line item in the Company’s condensed consolidated statements of operation:

	For the Three Months Ended June 30,
		2021		2020
	(in $000's)
Cost of revenue	$	1,030	$	705
Research and development		2,109		1,442
Selling, general and administrative		9,469		7,151
	$	12,608	$	9,298

The following table summarizes the stock option activity for the three months ended June 30, 2021:

						Weighted
				Weighted		Average		Aggregate
				Average		Remaining		Intrinsic
	Options			Exercise		Contractual		Value
	(in thousands)			Price		Term (years)		(in thousands)
Outstanding at beginning of period		711		$	141.87		5.46
Granted		60			285.52
Exercised		(56	)		38.02
Cancelled and expired		(3	)		259.61
Outstanding at end of period		712		$	161.56		5.77	$	111,794
Exercisable at end of period		562		$	135.54		4.89	$	103,757
Options vested and expected to vest at end of period		712		$	161.56		5.77	$	111,794

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, 2021, was approximately $12.8 million and the estimated weighted-average period over which this cost is expected to be recognized is 2.1 years.

The aggregate intrinsic value of stock options exercised was $15.7 million for the three months ended June 30, 2021. The total cash received as a result of employee stock option exercises for the three months ended June 30, 2021, was approximately $2.1 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 was as follows:

	For the Three Months Ended June 30,
		2021			2020
Weighted average grant-date fair value	$	103.03		$	75.75

Valuation assumptions:
Risk-free interest rate		0.79	%		0.31	%
Expected option life (years)		4.20			4.22
Expected volatility		44.28	%		42.80	%

The following table summarizes activity of restricted stock units for the three months ended June 30, 2021:

	Number of Shares			Weighted Average Grant Date Fair Value
	(in thousands)			(per share)
Restricted stock units at beginning of period		301		$	273.57
Granted		146			286.57
Vested		(84	)		303.33
Forfeited		(7	)		268.73
Restricted stock units at end of period		356		$	272.06

Restricted stock units generally vest annually over three years. The remaining unrecognized compensation expense for outstanding restricted stock units, including performance and market-based awards, as of June 30, 2021 was $81.0 million and the estimated weighted-average period over which this cost is expected to be recognized is 2.3 years.

The weighted average grant-date fair value for restricted stock units granted during the three months ended June 30, 2021 was $286.57. The total fair value of restricted stock units vested during the three months ended June 30, 2021 was $23.4 million.

In May 2021, performance-based awards of restricted stock units for the potential issuance of up to 44,778 shares of common stock were issued to certain executive officers and employees, which vest upon achievement of prescribed service milestones by the award recipients and the achievement of prescribed performance milestones by the Company. As of June 30, 2021, the Company is recognizing compensation expense based on the probable outcomes related to the prescribed performance targets on the outstanding awards.

In May 2020, the Company awarded certain executive officers and employees a total of up to 61,762 market-based restricted stock units. These restricted stock units will vest and result in the issuance of shares of common stock based on continuing employment and the relative ranking of the total shareholder return (“TSR”) of the Company’s common stock in relation to the TSR of twenty peer companies over a two-year and three-year performance period based on a comparison of average closing stock prices during the 20 trading days prior to the first day of the performance period, reinstated dividends during each performance period and the average closing stock prices during the final 20 trading days of each performance period. The actual number of market-based restricted stock units that may be earned can range from 0% to 200% of the target number of shares. Additionally, the payout percentage is further adjusted based on the Company’s performance relative to the constituents of the S&P 500 Index on the first day

of the performance period that are still actively trading on the last day of each performance period. The restricted stock units will vest following the end of the two-year and three-year performance period, respectively.

In May 2021, the Company awarded certain executive officers and employees a total of up to 62,930 market-based restricted stock units. These restricted stock units will vest upon achievement of prescribed service milestones by the award recipients and the achievement of prescribed performance milestones and relative TSR goals by the Company. These restricted stock units will vest after a single three-year period based upon performance and market milestones. The relative ranking of the TSR of the Company’s common stock in relation to the TSR of twenty peer companies over a three-year performance period based on a comparison of average closing stock prices during the 20 trading days prior to the first day of the performance period, reinstated dividends during each performance period and the average closing stock prices during the final 20 trading days of the performance period. The restricted stock units will vest following the end of the three-year performance period.

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 are recorded as compensation expense over the period from date of grant based on the probable outcomes related to the prescribed performance targets on the outstanding awards, regardless of the actual TSR outcome reached.

The table below sets forth the assumptions used to value the awards 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		1.9 - 2.9
Estimated fair value per share		$292.40		$347.05 - $349.28
Target performance (number of shares)		25,172			30,881