According to the expression status of three receptors: the estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor (HER2), breast cancer has been classified into Luminal A (LumA), Luminal B (LumB), epidermal growth factor receptor ERBB2/HER2-overexpressing (HER2+), and basal epithelial-like (BL). This evidence concerns the gene ESR1 and breast cancer.