The C4 Therapeutics, Inc. 2015 Incentive Stock Option and Grant Plan, or the 2015 Plan, adopted by the Company’s board of directors in December 2015 provides for the grant of incentive stock options, nonqualified stock options and restricted stock awards to eligible employees, outside directors and consultants of the Company.

In September 2020, the Company’s board of directors adopted the C4 Therapeutics, Inc. 2020 Stock Option and Incentive Plan, or the 2020 Plan. Following the effectiveness of the 2020 Plan, the Company ceased making grants under the 2015 Plan. However, the 2015 Plan continues to govern the terms and conditions of the outstanding awards granted under it. Shares of common stock subject to awards granted under the 2015 Plan that cease to be subject to such awards by forfeiture or otherwise after the termination of the 2015 Plan will be available for issuance under the 2020 Plan. As of March 31, 2022, the Company had 11,056,618 shares reserved under the 2020 Plan and 2015 Plan, and 3,735,127 shares available for future issuance under the 2020 Plan.

Stock-based compensation expense for the three months ended March 31, 2022 and 2021 was classified in the Company’s condensed consolidated statement of operations and comprehensive loss as follows (in thousands):

	Three Months Ended March 31,
	2022		2021
Stock-based compensation expense:
Research and development	$	3,612	$	1,171
General and administrative		5,326		2,674
Total stock-based compensation expense	$	8,938	$	3,845

The following table summarizes the stock option activity under the Company’s equity awards plans for the three months ended March 31, 2022:

	Number of Stock Options			Weighted-Average Exercise Price
Outstanding as of December 31, 2021		5,983,425		$	22.33
Granted		1,576,000			23.57
Exercised		(52,707	)		4.96
Cancelled or forfeited		(185,227	)		30.96
Outstanding as of March 31, 2022		7,321,491		$	22.72
Options exercisable as of March 31, 2022		1,668,108		$	18.45
Vested and expected to vest as of March 31, 2022		7,321,491		$	22.72

As of March 31, 2022, the unrecognized compensation cost related to outstanding options was $93.0 million, which is expected to be recognized over a weighted-average period of 3.0 years.

During the three months ended March 31, 2022, under the 2020 Plan, the Company’s Board of Directors authorized an issuance of restricted stock units to certain employees, including members of the Company’s leadership team. Stock units will vest in tranches as certain discovery milestones, clinical milestones, or specified market conditions are met. Upon vesting, each stock unit automatically converts into one share of the Company’s common stock.

The following table summarizes the restricted stock unit activity under the Company’s equity awards plans for the three months ended March 31, 2022:

	Shares		Weighted-Average Grant Date Fair Value
Outstanding as of December 31, 2021		—	$	—
Granted		563,500		25.47
Outstanding as of March 31, 2022		563,500	$	25.47

As of March 31, 2022, the unrecognized compensation cost related to outstanding restricted stock units was $13.8 million.