UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, DC 20549

SCHEDULE 13G

(Rule 13d-102)

Information to be Included in Statements Filed Pursuant

to § 240.13d-1(b), (c) and (d) and Amendments Thereto Filed

Pursuant to § 240.13d-2

Under the Securities Exchange Act of 1934

(Amendment No. 4)

ANNEXON, INC.

(Name of Issuer)

Common Stock, $0.001

par value per share

(Title of Class of Securities)

03589W102

(CUSIP Number)

September 30, 2024

(Date of Event Which Requires Filing of This Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

The information required in the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934, as amended (“Act”), or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

The name of the issuer to which this filing on Schedule 13G relates is Annexon, Inc. (the “Issuer”).

The principal executive offices of the Issuer are located at 1400 Sierra Point Parkway, Bldg C, Suite 200, Brisbane, California 94005.

This Schedule 13G is being filed jointly by Bain Capital Life Sciences Fund, L.P., a Cayman Islands exempted limited partnership (“BCLS Fund I”), BCIP Life Sciences Associates, LP, a Delaware limited partnership (“BCIPLS”), and BCLS I Investco, LP, a Delaware limited partnership (“BCLS I Investco” and, together with BCLS Fund I and BCIPLS, the “Reporting Persons”).

Bain Capital Life Sciences Investors, LLC, a Delaware limited liability company (“BCLSI”), is the general partner of Bain Capital Life Sciences Partners, LP, a Cayman Islands exempted limited partnership (“BCLSP”), which is the general partner of BCLS Fund I.

Boylston Coinvestors, LLC, a Delaware limited liability company (“Boylston”), is the general partner of BCIPLS. BCLSI governs the investment strategy and decision-making process with respect to investments held by BCIPLS.

BCLS I Investco GP, LLC, a Delaware limited liability company (“BCLS I Investco GP” and, together with the Reporting Persons, BCLSI, BCLSP and Boylston, the “Bain Capital Life Sciences Entities”), whose manager is BCLS Fund I, is the general partner of BCLS I Investco.

As a result, BCLSI may be deemed to share voting and dispositive power with respect to the securities held by the Reporting Persons.

The Reporting Persons have entered into a Joint Filing Agreement, dated November 14, 2024, pursuant to which the Reporting Persons have agreed to file this Schedule 13G jointly in accordance with the provisions of Rule 13d-1(k)(1) promulgated under the Act.

The principal business address for each of the Bain Capital Life Sciences Entities is 200 Clarendon Street, Boston, Massachusetts 02116.

BCLS Fund I and BCLSP are organized under the laws of the Cayman Islands. BCIPLS, BCLS I Investco, BCLSI, Boylston and BCLS I Investco GP are organized under the laws of the State of Delaware.

The class of securities of the Issuer to which this Schedule 13G relates is Common Stock, $0.001 par value per share (“Common Stock”).

The CUSIP number of the Common Stock is 03589W102.

If filing as a non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J), please specify the type of institution:

(a) Amount beneficially owned:

As of September 30, 2024, (i) BCLS Fund I held 2,245,213 shares of Common Stock, representing approximately 2.1% of the outstanding Common Stock, (ii) BCIPLS held 229,819 shares of Common Stock, representing approximately 0.2% of the outstanding Common Stock, and (iii) BCLS I Investco held 2,460,298 shares of Common Stock, warrants to purchase 774,943 shares of Common Stock and pre-funded warrants to purchase 2,739,475 shares of Common Stock, representing approximately 5.5% of the outstanding Common Stock.

The calculation of beneficial ownership of the Reporting Persons is based on (i) 105,653,962 shares of Common Stock issued and outstanding, as reported by the Issuer in its Quarterly Report on Form 10-Q for the quarter ended June 30, 2024, filed with the Securities and Exchange Commission on August 12, 2024 and (ii) 3,514,418 shares of Common Stock issuable upon the exercise of warrants and pre-funded warrants held by BCLS I Investco.

(b) Percent of class:

See Item 4(a) hereof.

(c) Number of shares as to which the person has:

(i) Sole power to vote or to direct the vote:

0 shares of Common Stock

(ii) Shared power to vote or to direct the vote:

See Item 4(a) hereof.

(iii) Sole power to dispose or to direct the disposition of:

0 shares of Common Stock

(iv) Shared power to dispose or to direct the disposition of:

See Item 4(a) hereof.

Not applicable.

Not applicable.

Not applicable.

Not applicable.

Not applicable.

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the Issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under § 240.14a-11.

SIGNATURES

After reasonable inquiry and to the best of its knowledge and belief, the undersigned certifies that the information in this statement is true, complete and correct.

Exhibit A

AGREEMENT REGARDING THE JOINT FILING OF

SCHEDULE 13G

The undersigned hereby agree as follows:

(i) Each of them is individually eligible to use the Schedule 13G to which this Exhibit is attached, and such Schedule 13G is filed on behalf of each of them; and

(ii) Each of them is responsible for the timely filing of such Schedule 13G and any amendments thereto, and for the completeness and accuracy of the information concerning such person contained therein; but none of them is responsible for the completeness or accuracy of the information concerning the other persons making the filing, unless such person knows or has reason to believe that such information is inaccurate.