According to the expression of estrogen receptor (ER), progesterone receptor (PR), and human epithelial growth factor receptor 2 (HER2), breast cancer can be classified into four categories: HER2−, ER+ and/or PR+; HER2+, ER+ and/or PR+; HER2+, ER−, PR−, and triple negative (HER2−, ER−, and PR−) (32). This evidence concerns the gene ESR1 and breast cancer.