MKI67 and breast cancer: Based on immunohistochemical evaluation of hormone receptors (including estrogen and progesterone receptors (ER and PR, respectively) and human epidermal growth factor receptor 2 (HER2)) and proliferation marker (Ki67), four molecular (intrinsic) invasive breast cancer subtypes were distinguished, namely, luminal A, luminal B, non-luminal HER2 positive and triple negative.