UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934*

(Amendment No. 2)

IGM Biosciences, Inc.

(Name of Issuer)

Common Stock, par value $0.01 per share

(Title of Class of Securities)

449585108

(CUSIP Number)

December 31. 2022

(Date of Event Which Requires Filing of this Statement)

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

☐ Rule 13d-1(b)

☒ Rule 13d-1(c)

☐ Rule 13d-1(d)

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 (“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).

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IGM Biosciences, Inc. (the “Issuer”)

325 E. Middlefield Road, Mountain View, CA 94043

(i) Invus Public Equities, L.P. (“Invus Public Equities”)

750 Lexington Avenue, 30th Floor, New York, NY 10022

Citizenship: Bermuda limited partnership

(ii) Invus Public Equities Advisors, LLC (“Invus PE Advisors”)

750 Lexington Avenue, 30th Floor, New York, NY 10022

Citizenship: Delaware limited liability company

(iii) Artal International S.C.A. (“Artal International”)

Valley Park, 44, Rue de la Vallée, L-2661, Luxembourg

Citizenship: Luxembourg limited partnership

(iv) Artal International Management S.A. (“Artal International Management”)

Valley Park, 44, Rue de la Vallée, L-2661, Luxembourg

Citizenship: Luxembourg société anonyme

(v) Artal Group S.A. (“Artal Group”)

Valley Park, 44, Rue de la Vallée, L-2661, Luxembourg

Citizenship: Luxembourg société anonyme

(vi) Westend S.A. (“Westend”)

Valley Park, 44, Rue de la Vallée, L-2661, Luxembourg

Citizenship: Luxembourg société anonyme

(vii) Stichting Administratiekantoor Westend (the “Stichting”)

Claude Debussylaan, 46, 1082 MD Amsterdam, The Netherlands

Citizenship: Netherlands foundation

(viii) Mr. Amaury Wittouck

Valley Park, 44, Rue de la Vallée, L-2661, Luxembourg

Citizenship: Belgium

The foregoing persons are hereinafter sometimes collectively referred to as the “Reporting Persons.”

Common Stock, par value $0.01 per share (the “Shares”)

449585108

Whether the Person Filing Is a(n):

This Item 3 is not applicable.

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(a) Amount beneficially owned:

As of December 31, 2022, Invus Public Equities directly held 2,450,000 Shares. Invus PE Advisors, as the general partner of Invus Public Equities, controls Invus Public Equities and, accordingly, may be deemed to beneficially own the Shares held by Invus Public Equities. The Geneva branch of Artal International, as the managing member of Invus PE Advisors, controls Invus PE Advisors and, accordingly, may be deemed to beneficially own the Shares that Invus PE Advisors may be deemed to beneficially own. Artal International Management, as the managing partner of Artal International, controls Artal International and, accordingly, may be deemed to beneficially own the Shares that Artal International may be deemed to beneficially own. Artal Group, as the sole stockholder of Artal International Management, controls Artal International Management and, accordingly, may be deemed to beneficially own the Shares that Artal International Management may be deemed to beneficially own. Westend, as the parent company of Artal Group, controls Artal Group and, accordingly, may be deemed to beneficially own the Shares that Artal Group may be deemed to beneficially own. The Stichting, as the majority stockholder of Westend, controls Westend and, accordingly, may be deemed to beneficially own the Shares that Westend may be deemed to beneficially own. Mr. Wittouck, as the sole member of the board of the Stichting, controls the Stichting and, accordingly, may be deemed to beneficially own the Shares that the Stichting may be deemed to beneficially own.

(b) Percent of class:

Each of the Reporting Persons may be deemed to be the beneficial owner of the percentage of Shares listed on such Reporting Person’s cover page. Calculations of the percentage of Shares beneficially owned are based on 29,138,628 Shares outstanding as of October 31, 2022 based on information set forth in the Issuer’s Quarterly Report on Form 10-Q filed with Securities and Exchange Commission on November 3, 2022.

(c) As of December 31, 2022, number of shares as to which Invus Public Equities, Invus PE Advisors, Artal International, Artal International Management, Artal Group, Westend, the Stichting and Mr. Wittouck has:

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

2,450,000

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

0

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

2,450,000

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

0

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following ☐.

On February 9, 2022, in connection with an internal reorganization, the Geneva branch of Artal International, the sole stockholder of Artal Treasury Limited, replaced Artal Treasury Limited as the managing member of Invus PE Advisors. Accordingly, Artal Treasury Limited is no longer deemed to beneficially own the Shares that Invus PE Advisors may be deemed to beneficially own and has ceased to be a Reporting Person.

This Item 6 is not applicable.

Reported on by the Parent Holding Company.

This Item 7 is not applicable.

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This Item 8 is not applicable.

This Item 9 is not applicable.

Each of the Reporting Persons hereby makes the following certification:

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.

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SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.