ERBB2 and breast cancer: BC can be classified into three main subfamilies according to the presence or absence of the hormone receptors for oestrogen (ER) and progesterone (PR) and the human epidermal growth factor receptor 2 (HER2) status: Luminal A/B (about 40%, ER+/PR+/-/HER2-), HER2+ (10-15%, ER-/PR-/HER2+) and those negative for all these marker, referred to as TNBC (15-20%, ER-/PR-/HER2-).