ERBB2 and breast cancer: Based on the presence or absence of commonly evaluated hormone receptors—estrogen (ER), progesterone (PR) and human epidermal growth factor 2 (HER2)—breast cancer is divided into four main subtypes: luminal A (ER+/PR+/HER2−), luminal B (ER+/PR+/HER2+), HER2 overexpressing or HER2-OE (ER−/PR+/HER2+) and triple-negative/basal-like type (ER−/ER−/HER2−) [1].