Notably, the immunosuppressive phenotype was associated with a high prevalence of mitogen-activated protein kinase kinase kinase 1 (MAP3K1) mutation, which suppressed MHC-I–mediated tumor antigen presentation by promoting the degradation of antigen peptide transporter 1/2 (TAP1/2) mRNAs, and thus reshaped the tumor immune microenvironment of HR+/HER2– breast cancers. Here, MAP2K1 is linked to breast cancer.