ERBB2 and breast cancer: While most patients with luminal A, ER+ HER− BC, are treated in the adjuvant setting with oral endocrine therapy and do not require chemotherapy, the mainstay of treatment for the luminal B, ER+ HER2+ BC subtype is chemotherapy with the monoclonal antibody (mAB) trastuzumab, administered before or after surgery [5,8].