Based on the estrogen receptor (ER) or progesterone receptor expression status and human epidermal growth factor 2 (HER2) gene amplification, breast cancer is divided into three major subgroups: hormone receptor+/HER2–, HER2+, and triple-negative breast cancer (TNBC) (Waks and Winer, 2019). This evidence concerns the gene ESR1 and breast cancer.