ERBB2 and breast cancer: The HER2-specific monoclonal antibody, trastuzumab [6, 7], transformed the treatment landscape for HER2+ BC, and together with subsequently developed HER2-targeting agents (pertuzumab, ado-trastuzumab emtansine, lapatinib, neratinib, trastuzumab deruxtecan, tucatinib, and margetuximab) are widely recommended by clinical guidelines for the treatment of patients with HER2+ BC [8–10].