Although anti-HER2 antibodies, including trastuzumab, pertuzumab, and the antibody–drug conjugates, trastuzumab emtansine (T-DM1, Kadcyla®), and trastuzumab deruxtecan (Enhertu®), are approved for treating HER2-overexpressing breast cancers, effective therapies are needed for patients who are refractory to HER2-targeting antibodies [2]. This evidence concerns the gene ERBB2 and breast cancer.