Over the past 20 years, anti-HER2 therapies such as trastuzumab, pertuzumab, lapatinib, trastuzumab-emtansine (T-DM1), trastuzumab deruxtecan (T-DXd), margetuximab, neratinib, and tucatinib revolutionized the treatment of metastatic HER2-overexpressing breast cancers. This evidence concerns the gene ERBB2 and breast cancer.