MKI67 and breast cancer: This heterogeneous disease can be classified into five molecular subtypes, luminal A, luminal B, human epidermal growth factor receptor 2 (HER2)-positive, basal-like, and normal-like BC, according to the expression of the estrogen receptor (ER) progesterone receptor (PR), the overexpression of HER2 cytokeratin CK, and the proliferation index Ki67 (Fig. 1).