MKI67 and cancer: Depending on HR and other proteins such as the human epidermal growth factor receptor 2- HER2, which are expressed (or not) in cancer cells, and the number of cells expressing the proliferation marker- Ki67, five molecular BC subtypes have been described: luminal A or HR+/HER2−/Ki67 low (HR-positive/HER2-negative/Ki67 < 20%), luminal B or HR+/HER2+/−Ki67 high (HR-positive/HER2-positive/negative/Ki67 ≥ 20%), triple-negative (TNBC) or HR−/HER2− (HR/HER2-negative), HER2-overexpressed, and so-called normal-like breast cancer which closely resembles luminal A subtype [7].