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 grant 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, 2021, the Company had 10,555,307 shares reserved under the 2020 Plan and 2015 Plan, and 4,602,393 shares available for future issuance under the 2020 Plan.

On March 3, 2020, the Company’s former president and chief executive officer, or the CEO, terminated employment with the Company. The Company repurchased all of the CEO’s exercised shares for total consideration of $0.1 million. The CEO also relinquished his right to purchase common shares through the exercise of vested options, for total consideration paid by the Company of $0.7 million. The Company recorded the repurchase liability once the termination was deemed probable during the three months ended March 31, 2020. The Company recognized the repurchase price of the common shares and the relinquishment of the vested options in additional-paid-in-capital on the condensed consolidated balance sheet as of March 31, 2020.

Stock-based compensation expense for the three months ended March 31, 2021 and 2020 was classified in the Company’s condensed consolidated statement of operations and comprehensive loss as follows (in thousands):

	Three Months Ended March 31,
	2021		2020
Stock-based compensation expense:
Research and development	$	1,171	$	116
General and administrative		2,674		—
Total stock-based compensation expense	$	3,845	$	116

The following table summarizes the stock option activity under the Company’s equity awards plans for the three months ended March 31, 2021:

	Number of Stock Options			Weighted-Average Exercise Price
Outstanding as of December 31, 2020		5,029,364		$	12.72
Granted		1,001,054			44.48
Exercised		(49,328	)		3.36
Cancelled or forfeited		(28,176	)		21.23
Outstanding as of March 31, 2021		5,952,914		$	18.10
Options exercisable as of March 31, 2021		531,248		$	4.47
Vested and expected to vest as of March 31, 2021		5,952,914		$	18.10

As of March 31, 2021, the unrecognized compensation cost related to outstanding options was $68.4 million, which is expected to be recognized over a weighted-average period of 3.6 years.