The breast cancer subtype determines standard systemic therapy, which includes hormonotherapy for the majority of hormone-receptor-positive (HR+) breast cancer (with some patients also requiring chemotherapy), trastuzumab-based ERBB2 antibody therapy plus chemotherapy for all ERBB2+ breast cancer (with additional endocrine therapy if concurrent HR-positive), and chemotherapy alone for triple-negative breast cancer [20]. This evidence concerns the gene ERBB2 and breast cancer.