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, 2023, there were 3,358,336 shares available for future grant 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, 2024, the annual increase for the 2020 Plan resulted in an additional 3,023,359 shares authorized for issuance being added to the 2020 Plan.


	Years Ended December 31,
	2023		2022
Stock-based compensation expense:
Research and development	$	11,826	$	13,108
General and administrative	15,409		16,908
Total stock-based compensation expense	$	27,235	$	30,016


	Number of Options	Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value (in thousands)
Outstanding as of December 31, 2022	6,993,052	$	21.57	7.78	$	1,031
Granted	4,180,270	3.87
Exercised	(13,096)	4.46
Forfeited/expired	(1,685,217)	18.48
Outstanding as of December 31, 2023	9,475,009	$	14.34	7.61	$	8,261
Exercisable as of December 31, 2023	3,838,707	$	20.54	6.68	$	482
Vested and expected to vest as of December 31, 2023	9,475,009	$	14.34	7.61	$	8,261


	Years Ended December 31,
	2023		2022
Weighted-average fair value of options granted	$	2.84	$	13.44
Intrinsic value of options exercised (in thousands)	$	22	$	1,796

As of December 31, 2023, the unrecognized compensation cost related to outstanding options was $51.6 million, which is expected to be recognized over a weighted-average period of 2.1 years.


	Years Ended December 31,
	2023	2022
Expected option life (years)	5.50 - 6.11	5.50 - 6.11
Risk-free interest rate	3.45% - 4.73%	1.47% - 4.22%
Expected volatility	81.99% - 86.15%	81.36% - 88.75%
Expected dividend yield	0.00%	0.00%

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


	Shares	Weighted-Average Grant Date Fair Value
Outstanding as of December 31, 2022	465,291	$	25.09
Granted	—	—
Vested (1)	(135,301)	26.52
Forfeited	(175,990)	28.91
Outstanding as of December 31, 2023	154,000	$	19.47

As of December 31, 2023, the unrecognized compensation cost related to outstanding PSUs was $3.1 million which is expected to be recognized over a weighted-average period of 1.2 years.

In January and February 2023, the Company’s Board of Directors authorized an issuance of 824,600 time-based restricted stock units, or RSUs, to employees under the 2020 Plan. Vesting of the RSUs are contingent to time-based vesting conditions. These RSUs are valued on the grant date using the grant date market price of the underlying shares. Upon vesting, each RSU automatically converts into one share of the Company’s common stock.


	Shares	Weighted-Average Grant Date Fair Value
Outstanding as of December 31, 2022	—	$	—
Granted	824,600	5.37
Vested	—	—
Forfeited	(74,390)	5.67
Outstanding as of December 31, 2023	750,210	$	5.35

As of December 31, 2023, the unrecognized compensation cost related to outstanding RSUs was $3.5 million which is expected to be recognized over a weighted-average period of 3.2 years.

On two separate dates in July 2023, the Company's Board of Directors approved the grant of non-qualified stock options to purchase an aggregate of 351,000 shares of the Company's common stock to two new employees. Options granted under these inducement grants vest over a period of four years with a cliff vesting at one year and quarterly vesting thereafter. As of December 31, 2023, none of these stock options have vested or been forfeited, and 351,000 options remain outstanding under these inducement awards.

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 issued 133,784 shares during the year ended December 31, 2023.

As of December 31, 2023, the Company had 1,673,129 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, 2024, the annual increase for the 2020 ESPP resulted in an additional 604,671 shares authorized for issuance being added to the 2020 ESPP.