The Company has issued stock-based awards from various equity incentive and stock purchase plans, as more fully described in Note 9, "Stock-Based Awards" to the consolidated financial statements in the Company's 2023 Annual Report on Form 10-K.

The following table summarizes stock option activity for the three months ended March 31, 2024:


	Number of Options	Weighted-Average Exercise Price
Balance at December 31, 2023	16,490,551	$	27.34
Granted	4,171,102	20.33
Exercised	(130,214)	7.85
Forfeited	(804,563)	28.50
Balance at March 31, 2024	19,726,876	$	25.94
Vested and expected to vest at March 31, 2024	18,917,005	$	27.02
Exercisable at March 31, 2024	11,826,599	$	26.70

The estimated fair value of stock options granted to employees were calculated using the Black-Scholes option-pricing model using the following assumptions:


	Three Months Ended March 31,
	2024	2023
Expected term (in years)	6.08	6.08
Volatility	64.5% - 64.6%	69.1% - 69.6%
Risk-free interest rate	3.9% - 4.0%	3.6% - 4.2%
Dividend yield	—	—

The following table summarizes restricted stock unit ("RSU") activity for the three months ended March 31, 2024:


	Number of RSU shares	Weighted-Average Fair Value at Date of Grant per Share
Unvested at December 31, 2023	3,635,157	$	35.60
Granted	1,919,193	20.23
Vested and released	(752,455)	39.78
Forfeited	(504,807)	28.86
Unvested and expected to vest at March 31, 2024	4,297,088	$	28.79

The Company’s results of operations include expenses relating to stock-based compensation as follows (in thousands):


	Three Months Ended March 31,
	2024		2023
Research and development	$	16,342	$	16,784
General and administrative	11,498		11,300
Total	$	27,840	$	28,084