Several classes of HER2-targeted agents have been developed for the treatment of HER2-positive (HER2+) breast cancer, including monoclonal antibodies (trastuzumab and pertuzumab), small-molecule tyrosine kinase inhibitors (lapatinib and neratinib) and antibody–drug conjugates (trastuzumab emtansine T-DM1) [2,3]. This evidence concerns the gene ERBB2 and breast cancer.