Clinically, breast cancer is grouped into luminal A (LA), luminal B (LB), human epidermal growth factor receptor 2-positive (HER2+), and triple-negative (TN) subtypes according to estrogen receptor (ER), progesterone receptor (PR), and epidermal growth factor receptor ErbB2/HER2 (HER2) expression. This evidence concerns the gene ESR1 and breast cancer.