Based on the surrogate markers estrogen receptor or progesterone receptor (ER/PR) status and human epidermal growth factor receptor (HER)-2 expression, breast cancer can be classified into ER+ (ER+/PR+ and HER-2−), ER+HER-2+ (ER+/PR+ and HER-2+), HER-2+ (ER−/PR− and HER-2+), and triple negative (ER−/PR− and HER-2−) [7, 8]. This evidence concerns the gene ESR1 and breast cancer.