ERBB2 and breast carcinoma: Human epidermal growth factor receptor 2 (HER2)-positive breast cancers (BCs) represent a distinct molecular subtype (~ 15–20% of all cases), defined by the overexpression of HER2 protein by immunohistochemistry (IHC) (IHC 3+) and/or amplification of HER2 gene by fluorescence in situ hybridization (FISH).