On December 28, 2015, the Company’s board of directors adopted the 2015 Incentive Stock Option and Grant Plan, or the 2015 Plan, and reserved 2,525,327 shares of common stock for issuance under this plan.

The 2015 Plan authorized the board of directors or a committee of the board to grant incentive stock options, nonqualified stock options and restricted stock awards to eligible employees, outside directors and consultants of the Company. Options granted under the 2015 Plan generally vest over a period of five or eight years with a cliff vesting at one year and quarterly vesting thereafter and options that lapse or are forfeited are available to be granted again. The contractual life of all options granted under the 2015 Plan is ten years from the date of grant.

On September 8, 2020, the Company’s board of directors adopted the C4 Therapeutics, Inc. 2020 Stock Option and Incentive Plan, or the 2020 Plan, which became effective on September 30, 2020. Upon adoption there were 6,567,144 shares of common stock reserved for issuance under the 2020 Plan. The Company’s Board of Directors, the Compensation Committee of the Board of Directors, and, in certain contexts, the Chief Executive Officer of the Company are authorized to grant a broad range of stock-based awards under the 2020 Plan, including stock options, stock appreciation rights, or SARs, restricted stock awards, or RSAs, restricted stock units, or RSUs, performance awards and stock bonus awards to the Company’s officers, employees, directors and other key persons, including consultants.

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 December 31, 2021, the Company had an aggregate of 9,704,418 shares reserved under the 2020 Plan and 2015 Plan, of which 3,720,993 shares were available for future issuance under the 2020 Plan.

The 2020 Plan provides for an annual increase, to be added on the first day of each fiscal year, beginning with January 1, 2021 and continuing until the expiration of the 2020 Plan, equal to the lesser of (i) 5% of the outstanding shares of common stock on the immediately preceding December 31st, or (ii) lesser number of shares determined by the administrator of the 2020 Plan, which is the Company’s Board of Directors or the Compensation Committee of the Board of Directors. On January 1, 2022, the annual increase for the 2020 Plan resulted in an additional 2,434,443 shares authorized for issuance being added to the 2020 Plan.

On March 3, 2020, or the Separation Date, the employment of the Company’s then current president and chief executive officer, or the Former CEO, terminated. The Company repurchased all of the Former CEO’s outstanding shares of common stock, which had been issued upon his exercise of previously granted stock options, for total consideration of $0.1 million. The Former CEO also relinquished his right to purchase shares of common stock upon the exercise of stock options that were vested as of his Separation Date, in exchange for total consideration paid by the Company of $0.7 million. The Company recognized the repurchase price of these shares of common stock and the relinquishment of these vested options in additional-paid-in-capital during the year ended December 31, 2020.

	Years Ended December 31,
	2021		2020		2019
Stock-based compensation expense:
Research and development	$	7,526	$	972	$	395
General and administrative		13,986		2,460		1,247
Total stock-based compensation expense	$	21,512	$	3,432	$	1,642

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

	Number of Options			Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding as of December 31, 2020		5,029,364		$	12.72		9.25	$	102,635
Granted		1,926,579			41.71
Exercised		(474,850	)		4.91
Forfeited/expired		(497,668	)		16.91
Outstanding as of December 31, 2021		5,983,425		$	22.33		8.64	$	75,538
Exercisable as of December 31, 2021		1,362,477		$	16.86		8.10	$	22,699
Vested and expected to vest as of December 31, 2021		5,983,425		$	22.33		8.64	$	75,538

	Years Ended December 31,
	2021		2020		2019
Weighted-average fair value of options granted	$	29.54	$	9.24	$	4.13
Intrinsic value of options exercised (in thousands)	$	17,321	$	2,587	$	337

As of December 31, 2021, the unrecognized compensation cost related to outstanding options was $74.5 million, which is expected to be recognized over a weighted-average period of 2.9 years.

The following table summarizes assumptions used in the Black-Scholes option pricing model to determine the fair value of stock options granted to employees:

	Years Ended December 31,
	2021	2020	2019
Expected option life (years)	5.50 - 6.11	5.23 - 6.35		6.35
Risk-free interest rate	0.59% - 1.34%	0.32% - 0.57%	1.71% - 2.36%
Expected volatility	80.45% - 86.20%	69.54% - 83.76%	65.50% - 76.80%
Expected dividend yield	0.00%	0.00%	0.00%

In September 2020, the Company’s Board of Directors adopted the C4 Therapeutics, Inc. 2020 Employee Stock Purchase Plan, or the 2020 ESPP. Eligible employees may authorize payroll deductions of up to 15% of their eligible compensation during an offering period. The Company may hold one or more offering periods each year during which employees will be able to purchase shares under the 2020 ESPP. The Company held its first two six-month offering periods during the year ended December 31, 2021 and issued 6,640 shares.

As of December 31, 2021, the Company had 861,765 shares available for future issuance under the 2020 ESPP. The 2020 ESPP provides for an annual increase to be added on the first day of each fiscal year, beginning with January 1, 2021 and continuing thereafter through January 1, 2030, equal to the lesser of (i) 1% of the outstanding shares of common stock on the immediately preceding December 31st, (ii) 656,714 shares, or (ii) lesser number of shares determined by the administrator of the 2020 ESPP. On January 1, 2022, the annual increase for the 2020 ESPP resulted in an additional 486,888 shares authorized for issuance being added to the 2020 ESPP.

In January and February 2022, the Company’s Board of Directors authorized an issuance of 563,500 RSUs to certain employees, including members of the Company’s leadership team under the 2020 Plan. Vesting of the RSUs are contingent upon the determination of achievement of certain discovery and clinical milestones, or as specified market conditions are met. Upon vesting, each RSU converts automatically into one share of the Company’s common stock.