The Company's 2015 Long-Term Incentive Plan ("2015 Stock Plan") and 2021 Incentive Award Plan (“2021 Stock Plan” and, together, “Stock Plans”) permit the grant of restricted stock units (“RSUs”), stock options, and other stock-based awards to employees, non-employee directors, and consultants.

Stock option activity during the three months ended March 31, 2024 was not material. The following table summarizes the Company’s restricted stock unit activity during the three months ended March 31, 2024:


	RSUs
		Number of Shares (in millions)	Weighted-Average Grant-Date Fair Value
Outstanding at December 31, 2023		56	$	22.36
Granted		22	$	12.58
Exercised / Vested		(25)	$	15.18
Forfeited / Cancelled		(2)	$	22.99

Outstanding at March 31, 2024		51	$	21.73
Vested and expected to vest at March 31, 2024		51	$	21.73

The following table summarizes the Company’s stock-based compensation expense for the Stock Plans and 2021 Employee Stock Purchase Plan (“ESPP”) by line item in the Condensed Consolidated Statements of Operations (in millions):




	Three Months Ended March 31,
		2023		2024
Cost of revenues		$	18	$	23
Research and development		84		124
Selling, general, and administrative		81		86
Total stock-based compensation expense		$	183	$	233

As of March 31, 2024, the Company’s unrecognized stock-based compensation expense for unvested awards was $1,233 million, which is expected to be recognized over a weighted-average period of 5.5 years and 1.7 years for stock options and RSUs outstanding, respectively.