Exhibit 99.1

IGM Biosciences Announces Third Quarter 2021 Financial Results and Provides Corporate Update

– Plans to Initiate Phase 2 Studies of IGM-2323 in Diffuse Large B-cell Lymphoma and Follicular Lymphoma –

– Data from IGM-2323 Selected for Oral Presentation at 2021 ASH Annual Meeting –

– IGM-8444 Successfully Completes Phase 1 Monotherapy Dose Escalation –

– IGM-8444 Showed No Clinically Significant Liver Toxicity or Dose Limiting Toxicities Observed to Date –

– IGM-2644 (CD38 x CD3) Expected to Enter Clinical Testing for Multiple Myeloma in 2022 –

– IGM Establishes Infectious Diseases and Autoimmunity and Inflammation Business Units –

MOUNTAIN VIEW, Calif., Nov. 4, 2021 (GLOBE NEWSWIRE) — IGM Biosciences, Inc. (Nasdaq: IGMS), a clinical-stage biotechnology company focused on creating and developing engineered IgM antibodies, today announced its financial results for the third quarter ended September 30, 2021 and provided an update on recent developments.

“IGM continues to validate and expand the IgM platform through the clinical development of IGM-2323, our CD20 x CD3 T cell engager IgM antibody for the treatment of B cell proliferative diseases, IGM-8444, our Death Receptor 5 (DR5) agonist IgM antibody for the treatment of solid and hematologic cancers, and the establishment of our infectious diseases and autoimmunity and inflammation business units,” said Fred Schwarzer, Chief Executive Officer of IGM Biosciences. “We plan to continue to expand our clinical development efforts, and by the end of 2022 we expect to be actively pursuing the clinical development of four oncology product candidates, led by two Phase 2 clinical studies of IGM-2323.”

“We are encouraged by the emerging data from the clinical testing of IGM-2323 in our most fully explored titration dose cohort, 100 mg, where we have seen multiple complete responses in both diffuse large B-cell lymphoma (DLBCL) and follicular lymphoma (FL), and we look forward to sharing the initial data from this 100 mg dose cohort at our American Society of Hematology (ASH) presentation in December,” said Chris Takimoto, M.D., Ph.D., F.A.C.P., Chief Medical Officer of IGM Biosciences. “While the number of patients we have treated at 100 mg is relatively small, we believe that the planned Phase 2 expansion studies could potentially provide the basis for accelerated review and approval. As a first step, consistent with the spirit of the U.S. Food and Drug Administration’s (FDA) Project Optimus, we plan to test doses of 100 mg and 300 mg in two separate randomized ‘pick the winner’ Phase 2 studies of 30 patients at each dose level, one in DLBCL and one in FL. We plan to then expand the Phase 2 studies at the optimal dose, while including the prior data from that dose for purposes of potential registration. While our safety profile at 600 mg and 1000 mg is consistent with lower doses and very encouraging, we do not plan to continue to explore doses higher than 300 mg for purposes of greater efficacy in either DLBCL or FL.”

Pipeline Updates

IGM-2323 (CD20 x CD3)

As described in the abstract released today, as of April 30, 2021, 29 patients had been enrolled in the Phase 1 study of IGM-2323: 12 at 5 fixed dose levels (0.5, 2.5, 10, 20, 100 mg) and 17 at 5 dose titration levels (100, 200, 300, 600, and 1000 mg). All 29 patients received at least one dose and were evaluable for safety. There were no dose limiting toxicities (DLTs) and no neurotoxicity adverse events (AEs). No patients discontinued due to an AE. Of the 29 patients evaluable for safety, 6 patients had cytokine release syndrome (CRS), primarily Grade 1. As previously described, there were only two higher grade CRS events as of April 30, 2021, one Grade 2 and one Grade 3. As previously described, the Grade 3 patient had been treated with an experimental CAR-T and had high baseline circulating B-cells. Of the 11 evaluable patients treated in the titration dose cohorts, as of the April 30, 2021 data cut-off, there were 5 responses, 3 complete responses and 2 partial responses.

IGM-8444 (DR5)

“IGM-8444’s toxicity profile to date, which has not shown any clinically significant liver toxicity, differentiates it from some of the second generation DR5 agonists that have struggled to progress in the clinic due to liver toxicity,” said Dr. Takimoto. “Importantly, we believe this safety profile will be critical to successful combinations with other drugs, which we believe represent by far the most exciting and promising uses of DR5 agonists for the treatment of multiple solid and hematologic cancers. For this reason, our clinical development focus continues to be on combinations of IGM-8444 with standard of care and novel agents, such as FOLFIRI, venetoclax and birinapant.”

IGM-6268 (COVID-19)

IGM-7354 (IL-15 x PD-L1)

IGM-2644 (CD38 x CD3)

“We believe the safety and efficacy profile that we have observed to date in the clinical development of IGM-2323, our T cell engaging IgM antibody targeting CD20 on lymphoma cells, is very encouraging with respect to the future clinical development of IGM-2644, our T cell engaging IgM antibody targeting CD38 on multiple myeloma cells, as it has shown similar preclinical safety and efficacy features to those we observed with IGM-2323,” said Bruce Keyt, Ph.D., Chief Scientific Officer of IGM Biosciences. “We hope to file an investigational new drug (IND) application with the FDA and begin the Phase 1 clinical development of IGM-2644 next year.”

Corporate Updates

Third Quarter 2021 Financial Results

2021 Financial Guidance

IGM reiterates its previously issued financial guidance expecting full year GAAP operating expenses to be between $175 million and $185 million including estimated non-cash stock-based compensation expense of approximately $25 million. IGM expects to end 2021 with a balance of over $200 million in cash and investments.

About IGM Biosciences, Inc.

Headquartered in Mountain View, California, IGM Biosciences is a clinical-stage biotechnology company focused on creating and developing engineered IgM antibodies. Since 2010, IGM Biosciences has worked to overcome the manufacturing and protein engineering hurdles that have limited the therapeutic use of IgM antibodies. Through its efforts, IGM Biosciences has created a proprietary IgM technology platform for the development of IgM antibodies for those clinical indications where their inherent properties may provide advantages as compared to IgG antibodies.

Cautionary Note Regarding Forward-Looking Statements

This press release contains forward-looking statements, including statements relating to IGM’s plans, expectations and forecasts and to future events. Such forward-looking statements include, but are not limited to: the potential of, and expectations regarding IGM’s technology platform and its IgM and IgA antibodies and product candidates, including IGM-2323, IGM-8444, IGM-6268, IGM-7354 and IGM-2644; IGM’s plans and expectations regarding its clinical development efforts and activities; statements regarding the clinical development of IGM-2323, including IGM’s plans to initiate two Phase 2 studies of IGM-2323, the design of such studies, expectations regarding dosing and the potential for accelerated approvals and such Phase 2 studies serving as potentially registrational studies; statements regarding the clinical development of IGM-8444, including IGM’s plans to enroll patients with chronic lymphocytic

leukemia/small lymphocytic lymphoma in a venetoclax-IGM-8444 combination cohort, the safety profile of IGM-8444 and its potential for use in combination with other drugs for the treatment of multiple solid and hematologic cancers; plans to initiate Phase 1 studies of IGM-6268, IGM-7354 and IGM-2644, and the expected timing of the initiation of such studies and the filing of an IND with the FDA for IGM-2644; IGM’s expansion into infectious diseases and autoimmunity and inflammation, and its newly created infectious diseases and autoimmunity and inflammation business units; the Company’s research and development strategy; IGM’s expectations regarding its financial position, including operating expenses, cash and investments and non-cash stock-based compensation; and statements by IGM’s Chief Executive Officer, Chief Medical Officer and Chief Scientific Officer. Such statements are subject to numerous important factors, risks and uncertainties that may cause actual events or results to differ materially, including but not limited to: potential delays and disruption resulting from the COVID-19 pandemic and governmental responses to the pandemic, including any future impacts to IGM’s operations, the manufacturing of its product candidates, the progression of its clinical trials, enrollment in its current and future clinical trials and progression of its collaborations and related efforts; IGM’s early stages of clinical drug development; risks related to the use of engineered IgM antibodies, which is a novel and unproven therapeutic approach; IGM’s ability to demonstrate the safety and efficacy of its product candidates; IGM’s ability to successfully and timely advance its product candidates through preclinical studies and clinical trials; IGM’s ability to enroll patients in its clinical trials; the potential for the results of clinical trials to differ from preclinical, preliminary, initial or expected results; the risk of significant adverse events, toxicities or other undesirable side effects; IGM’s ability to successfully manufacture and supply its product candidates for clinical trials; the potential impact of continuing or worsening supply chain constraints; the risk that all necessary regulatory approvals cannot be obtained; the potential market for IGM’s product candidates, the potential diminishing need for therapeutics to address COVID-19, particularly in the United States and other major markets, and the progress and success of alternative therapeutics currently available or in development; IGM’s ability to obtain additional capital to finance its operations, if needed; uncertainties related to the projections of the size of patient populations suffering from the diseases IGM is targeting; IGM’s ability to obtain, maintain and protect its intellectual property rights; developments relating to IGM’s competitors and its industry, including competing product candidates and therapies; risks related to collaborations with third parties, including the risk of the occurrence of any event, change or other circumstance that could give rise to the termination of any such collaboration; general economic and market conditions; and other risks and uncertainties, including those more fully described in IGM’s filings with the Securities and Exchange Commission (SEC), including IGM’s Annual Report on Form 10-K filed with the SEC on March 30, 2021, IGM’s Quarterly Report on Form 10-Q filed with the SEC on November 4, 2021 and in IGM’s future reports to be filed with the SEC. Any forward-looking statements contained in this press release speak only as of the date hereof, and IGM specifically disclaims any obligation to update any forward-looking statement, except as required by law.

Contact

Argot Partners

David Pitts

212-600-1902

igmbio@argotpartners.com

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IGM Biosciences, Inc.

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IGM Biosciences, Inc.

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