ERBB2 and breast cancer: In clinical practice, according to the expression levels of estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), and the proliferation index Ki67 which has connection with ER and PR expression, breast cancer consists of four related types, namely luminal A, luminal B, HER2 and triple negative breast cancer (TNBC) (Sørlie et al. 2001).