Based on the presence or absence of the estrogen-receptor (ER), progesterone-receptor (PR), and human epidermal growth factor receptor 2 (HER2) status, BC can be classified into three clinical subtypes: hormone-receptor (HR) positive (HR+; ER+, PR+/−, and HER2−), HER2 positive (HER2+), and triple negative (TN; ER−, PR−, and HER2−) [3]. Here, ESR1 is linked to breast cancer.