SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

AMENDMENT NO. 1

TO

SCHEDULE TO

Tender Offer Statement Under Section 14(d)(1) or 13(e)(1)

of the Securities Exchange Act of 1934

IGM BIOSCIENCES, INC.

(Name of Subject Company (Issuer) and Filing Person (Offeror))

Options to Purchase Common Stock Covering Common Stock, $0.01 par value

(Title of Class of Securities)

449585108

(CUSIP Number of Class of Securities’ Underlying Common Stock)

Fred M. Schwarzer

Chief Executive Officer and President

325 E. Middlefield Road

Mountain View, CA 94043

(650) 965-7873

(Name, address and telephone numbers of person authorized to receive notices and communications on behalf of filing persons)

Copies to:

Check the appropriate boxes below to designate any transactions to which the statement relates:

Check the following box if the filing is a final amendment reporting the results of the tender offer: ☐

If applicable, check the appropriate box(es) below to designate the appropriate rule provision(s) relied upon:

INTRODUCTION

This Amendment No. 1 (this “Amendment”) amends and supplements the Tender Offer Statement on Schedule TO (together with any subsequent amendments and supplements thereto, the “Schedule TO”) originally filed with the Securities and Exchange Commission by IGM Biosciences, Inc., a Delaware corporation (“IGM” or the “Company”) on June 20, 2024. The Schedule TO relates to an offer by the Company to exchange certain outstanding stock options to purchase shares of the Company’s common stock, on the terms set forth in the Offer to Exchange Certain Outstanding Options for Restricted Stock Units dated June 20, 2024 (the “Offer to Exchange”), previously filed as Exhibit (a)(1)(A) to the Schedule TO.

This Amendment is being filed to amend and supplement certain provisions of the Schedule TO as set forth herein. Except as specifically provided herein, the information contained in the Schedule TO and the Offer to Exchange remains unchanged. This Amendment should be read in conjunction with the Schedule TO and the Offer to Exchange.

Amendments to the Offer to Exchange

The Offer to Exchange and Items 1 through 11 of the Schedule TO, to the extent such items incorporate by reference the information contained in the Offer to Exchange, are hereby amended and supplemented as follows:

Offer to Exchange – Summary Term Sheet and Questions and Answers

On page 1 of the Offer to Exchange, the sixth bullet under the heading “Summary Term Sheet and Questions and Answers—Q1. What is the offer? —Terms Used in This Offer to Exchange” is amended and restated as follows:

On page 6 of the Offer to Exchange, the first paragraph under the heading “Summary Term Sheet and Questions and Answers—Q7. Which of my options are eligible?” is amended and restated as follows:

“A7. Your eligible options are those options to purchase shares of common stock of IGM that were granted on or prior to March 1, 2023 under the 2018 Plan, have a per share exercise price equal to or greater than $17.70, whether vested or unvested, remain outstanding and unexercised as of the expiration date, currently expected to occur on July 18, 2024, and have a per share exercise price greater than the closing price of our common stock on the expiration date, currently expected to occur on July 19, 2024. The determination of which options are eligible was made by our board of directors, with the advice of external advisors, based on a careful balancing of a number of factors.”

Offer to Exchange – 2. Number of RSUs; expiration date.

On page 99 of the Offer to Exchange, the first paragraph under the heading “Number of RSUs; expiration date” is amended and restated as follows:

“Subject to the terms and conditions of this offer, we will accept for exchange options granted on or prior to March 1, 2023 with a per share exercise price equal to or greater than $17.70, whether vested

or unvested, that were granted under the 2018 Plan, are held by eligible employees, are outstanding and unexercised as of the expiration date of the offer, have a per share exercise price greater than the closing price of our common stock on the expiration date, are properly elected to be exchanged, and are not validly withdrawn before the expiration date of the offer. In order to be eligible, options must be outstanding on the expiration date of the offer. For example, if a particular option grant expires during the offering period, that option grant is not eligible for exchange.”

Offer to Exchange – 7. Conditions of the offer.

On page 110 of the Offer to Exchange, the third bullet under the heading “Conditions of the offer” is amended and restated as follows:

“• Any of the following:

On page 111 of the Offer to Exchange, the final paragraph under the heading “Conditions of the offer” is amended and restated as follows:

“The conditions to this offer are for our benefit. In our discretion, we may assert them before the expiration date regardless of the circumstances giving rise to them (other than circumstances caused by our action or inaction). We may waive any condition, in whole or in part, at any time and from time to time before the expiration date, in our discretion, whether or not we waive any other condition to the offer. Any such waiver will apply to all eligible employees in a uniform and non-discretionary manner. Our failure at any time to exercise any of these rights will not be deemed a waiver of any such rights, but will be deemed a waiver of our ability to assert the condition that was triggered with respect to the particular circumstances under which we failed to exercise our rights. Any determination we make concerning the events described in this Section 7 will be given the maximum deference permitted by law. However, you have all rights accorded to you under applicable law to challenge such determination in a court of competent jurisdiction. Only a court of competent jurisdiction can make a determination that will be final and binding upon the parties.”

Offer to Exchange – 8. Price range of shares underlying the options.

On page 112 of the Offer to Exchange, the table under the heading “Price range of shares underlying the options” is amended and restated as follows:

Offer to Exchange – 9. Source and amount of consideration; terms of RSUs.

On page 113 of the Offer to Exchange, the second paragraph under the heading “Source and amount of consideration; terms of RSUs—General terms of RSUs” is amended and restated as follows:

“The following description summarizes the material terms of the 2018 Plan. Our statements in this Offer to Exchange concerning the 2018 Plan and the RSUs are subject to, and are qualified in their entirety by reference to the 2018 Plan and the form of RSU award agreement under the 2018 Plan, which is available on the SEC website at www.sec.gov. The form of RSU award agreement under the 2018 Plan is incorporated by reference as an exhibit to the Schedule TO with which this Offer to Exchange has been filed. In addition, a copy of the 2018 Plan and the form of RSU award agreement are available on the SEC website at www.sec.gov. To receive a copy of the 2018 Plan and/or the form of RSU award agreement please contact:

Infinite Equity

Email: IGM@infiniteequity.com

We will promptly furnish to you copies of these documents upon request at our expense.”

Item 12. Exhibits.

Item 13. Information Required by Schedule 13E-3.

(a) Not applicable.

SIGNATURE

After due inquiry and to the best of my knowledge and belief, I certify that the information set forth in this Amendment No. 1 to Schedule TO is true, complete and correct.

Date: July 1, 2024

INDEX TO EXHIBITS