HER2 overexpression is associated with poor prognosis, but HER2+ BC patients can be treated with HER2-targeted therapies, such as the monoclonal antibodies trastuzumab and pertuzumab, the antibody-drug conjugate trastuzumab emtansine (T-DM1), and the tyrosine kinase inhibitor lapatinib, alone or in combination with chemotherapy (see Table 1). This evidence concerns the gene ERBB2 and breast cancer.