Clinically, breast cancer is categorized into three major subtypes: (1) hormone receptor-positive, which refers to the presence of the estrogen receptor alpha (ER, encoded by the gene ESR1) and/or progesterone receptor (PR); (2) human epidermal growth factor receptor 2 (HER2)-positive, which refers to the overexpression of HER2 due to amplification of the ERBB2 locus; and (3) triple negative breast cancer (TNBC), which lacks expression of the above-mentioned markers [2]. Here, ESR1 is linked to breast cancer.