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 nine months ended September 30, 2024 was not material. The following table summarizes the Company’s restricted stock unit activity during the nine months ended September 30, 2024:


	RSUs
		Number of Shares (in millions)	Weighted-Average Grant-Date Fair Value
Outstanding at December 31, 2023		56	$	22.36
Granted		72	$	10.47
Exercised / Vested		(48)	$	16.28
Forfeited / Cancelled		(12)	$	19.49

Outstanding at September 30, 2024		68	$	14.66
Vested and expected to vest at September 30, 2024		68	$	14.66

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 September 30,				Nine Months Ended September 30,
	2023		2024		2023		2024
Cost of revenues	$	23	$	6	$	64	$	46
Research and development	133		59		289		281
Selling, general, and administrative	86		46		253		211
Total stock-based compensation expense	$	242	$	111	$	606	$	538

Stock-based compensation expense decreased in the three and nine months ended September 30, 2024 primarily related to a reversal in the accrual of a previously recorded stock-based bonus expense.

As of September 30, 2024, the Company’s unrecognized stock-based compensation expense for unvested awards was $995 million, which is expected to be recognized over a weighted-average period of 5.1 years and 1.7 years for stock options and RSUs outstanding, respectively.