Breast cancer (BC), a disease of high heterogeneity, is divided into four molecular subtypes (luminal A, luminal B, human epidermal growth factor receptor 2- (HER2-) enriched, and triple negative) according to immunohistochemistry (IHC) markers, such as receptor expression of estrogen (ER) and progesterone (PR), HER-2 expression status, and Ki67 index 1. The gene discussed is MKI67; the disease is breast cancer.