MKI67 and breast cancer: The breast cancer (BC) subtypes, as determined by either gene expression analysis or surrogate immunohistochemical (IHC) markers (oestrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), proliferation marker (Ki67), and Nottingham histological grade (NHG), have different prognostic and predictive characteristics [1, 2].