ERBB2 and breast cancer: In human epidermal growth factor receptor 2 (HER2)-overexpressing metastatic or advanced breast cancer (BC), defined as: immunohistochemistry (IHC) 3+ or positive by in situ hybridization (ISH), treatments with HER2-targeting monoclonal antibodies trastuzumab, pertuzumab and the antibody-drug conjugate trastuzumab emtansine (TDM-1) have demonstrated significantly increased response rates and prolongation of progression-free survival (PFS) and overall survival [1–6].