Annexon, Inc. Stock Compensation Disclosure

In July 2020, the Company’s board of directors and stockholders adopted and approved the 2020 Incentive Award Plan, or the 2020 Plan, and the Employee Stock Purchase Plan, or the ESPP, which became effective in connection with the IPO.

The Company may not grant any additional awards under the 2011 Equity Incentive Plan, or the 2011 Plan. The 2011 Plan will continue to govern outstanding equity awards granted thereunder.

2020 Equity Incentive Plan

The number of shares of common stock reserved for issuance under the 2020 Plan automatically increase on the first day of January, in an amount equal to 4% of the total number of shares of the Company’s capital stock outstanding on the last day of the preceding year, or a lesser number of shares determined by the Company’s board of directors.

Awards granted under the 2020 Plan expire no later than ten years from the date of grant. For the Incentive Stock Options, or ISOs, and Nonstatutory Stock Options, or NSOs, the option price shall not be less than 100% of the estimated fair value on the date of grant. Options granted typically vest over a four-year period but may be granted with different vesting terms. As of December 31, 2025 and 2024, there were 1,184,877 and 2,109,758 shares available for issuance under the 2020 Plan, respectively.

2022 Employment Inducement Award Plan

In July 2022, the Company’s board of directors adopted the Annexon, Inc. 2022 Employment Inducement Award Plan, or the Inducement Plan, and together with the 2011 Plan and the 2020 Plan, the Plans. The Inducement Plan was adopted by the Company’s board of directors without stockholder approval pursuant to Nasdaq Marketplace Rule 5635(c)(4), or Rule 5635(c)(4). In accordance with Rule 5635(c)(4), awards made under the Inducement Plan may only be granted to newly hired employees as an inducement material to the employees entering into employment with the Company. Awards granted under the Inducement Plan expire no later than ten years from the date of grant. An aggregate of 7,850,000 shares of common stock were reserved for issuance under the Inducement Plan. As of December 31, 2025 and 2024, there were 3,585,793 and 3,359,230 shares available for issuance under the Inducement Plan, respectively.

Stock options

The total intrinsic value of options exercised during the years ended December 31, 2025 and 2024 was $0.5 million and $1.2 million, respectively. The intrinsic value is the difference between the fair value of the Company’s common stock at the time of exercise and the exercise price of the stock option.

The weighted-average grant date fair value of options granted to employees during the years ended December 31, 2025 and 2024 was $1.99 and $4.56 per share, respectively.

As of December 31, 2025, the total unrecognized stock-based compensation cost related to outstanding unvested stock options was $24.5 million, which the Company expects to recognize over an estimated weighted-average period of 2.6 years.

Restricted Stock Units

RSUs are share awards that entitle the holder to receive freely tradeable shares of the Company’s common stock upon vesting. The RSUs cannot be transferred and the awards are subject to forfeiture if the holder’s employment

terminates prior to the release of the vesting restrictions. The RSUs generally vest over a three-year period in equal amounts on an annual basis, provided the employee remains continuously employed with the Company. The fair value of the RSUs is equal to the closing price of the Company’s common stock on the grant date.

As of December 31, 2025, unrecognized stock-based compensation expense related to outstanding unvested RSUs was $2.6 million, which is expected to be recognized over a weighted-average period of 1.8 years.

Employee Stock Purchase Plan

The ESPP enables eligible employees to purchase shares of the Company’s common stock at the end of each offering period at a price equal to 85% of the fair market value of the shares on the first business day or the last business day of the offering period, whichever is lower. Eligible employees generally included all employees. Share purchases are funded through payroll deductions of at least 1%, and up to 15% of an employee’s eligible compensation for each payroll period. The number of shares reserved for issuance under the ESPP increase automatically on the first day of each fiscal year, by a number equal to, 1% of the shares of common stock outstanding on the last day of the immediately preceding fiscal year, or such number of shares determined by the Company’s board of directors. As of December 31, 2025, 2,800,550 shares were available for future purchase. The ESPP generally provides for six-month consecutive offering periods beginning on May 15th and November 15th of each year. The ESPP is a compensatory plan as defined by the authoritative guidance for stock compensation. As such, stock-based compensation expense has been recorded for the years ended December 31, 2025 and 2024.

The stock-based compensation expense related to the ESPP for each of the years ended December 31, 2025 and 2024 was $0.2 million.

Stock-Based Compensation Expense

To determine the value of stock option awards for stock-based compensation purposes, the Company uses the Black-Scholes option pricing model and the assumptions discussed below. Each of these inputs is subjective and generally requires significant judgment.

Fair Value of Common Stock—The fair value of each share of underlying common stock is based on the closing price of the Company’s common stock as reported on the date of grant on the Nasdaq Global Select Market.

Expected Term—The expected term represents the period that the stock-based awards are expected to be outstanding and is determined using the simplified method. The Company continues using the simplified method as it

does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate expected term due to the limited period of time its equity shares have been publicly traded.

Expected Volatility—Beginning in 2024, the expected volatility was estimated based on a weighted volatility using both the Company’s trading history for its common stock and the average volatility for comparable publicly traded life sciences companies over a period equal to the expected term of the stock option grants. The comparable companies were chosen based on the similar size, stage in life cycle or area of specialty.

Risk-Free Interest Rate—The risk-free interest rate is based on the U.S. Treasury zero coupon issues in effect at the time of grant for periods corresponding with the expected term of the option.

Dividend Yield—The Company has never paid dividends on its common stock and has no plans to pay dividends on its common stock. Therefore, the Company used an expected dividend yield of zero.