There are five main molecular subtypes of BC based on the inherency of the estrogen (ER) [4], progesterone (PR) [7] and human epidermal growth factor (HER2) [8] receptors, as well as the intensity of proliferation index-67 (Ki-67) [9], including luminal A (ER+/PR+; HER2−; Ki67−) [10], luminal B ((ER+/PR+; HER2−; Ki67+)/(ER+/PR+; HER2+; Ki67+)) [11], HER2-overexpressed (ER−/PR−; HER2+) [12], triple-negative breast cancer (TNBC) (ER−/PR−; HER2−) [13] and normal-like BC (ER+/PR+; HER2−; Ki67−) [14,15]. This evidence concerns the gene MKI67 and breast cancer.