Since its discovery in the 1990s, several agents have been developed and approved for the treatment of HER2-positive breast cancer: monoclonal antibodies, pertuzumab and margetuximab; tyrosine kinase inhibitors, lapatinib, neratinib, and tucatinib, small molecules that bind the intracellular domain of HER2 and block its activity; and antibody-drug conjugates, ado-trastuzumab emtansine (T-DM1) and trastuzumab deruxtecan (T-DXd). This evidence concerns the gene ERBB2 and breast cancer.