Breast cancer is a heterogeneous disease and can be classified into at least 5 subtypes: (1) luminal A (usually ER and/or PR+, HER2−, low Ki67), (2) luminal B (usually ER+and/or PR+, HER2+, or HER2− with high Ki67), (3) HER2 positive, (4) basal like, and (5) normal breast like1, 2. The gene discussed is MKI67; the disease is breast cancer.