Breast cancer can be classified into four molecular subgroups according to how the estrogen receptor (ER), the progesterone receptor (PR), and the human epidermal growth factor receptor 2 (HER2) are expressed: luminal A (ER+ and/or PR+, and HER2−), luminal B (ER+ and/or PR+, and HER2+), HER2+ (ER−, PR−, and HER2+), and triple-negative (TNBC: ER−, PR−, and HER2−) [7]. Here, ESR1 is linked to breast cancer.