The molecular classification of breast cancers is as follows: luminal A: the majority are ER-positive, HER2-negative tumors with a low Ki67 index; luminal B: the majority are ER-positive, HER2-positive tumors with a high Ki67 index; HER2-enriched ER-negative tumors with HER2 overexpression; and basal-like tumors with ER and HER2 negativity. This evidence concerns the gene MKI67 and breast cancer.