According to the IHC/fluorescence in situ hybridization (FISH) profile, BC can be classified and divided on the basis of presence of the estrogen receptor (ER), the progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) into ER-positive, HER2-positive, and triple-negative BC (TNBC) that is defined by the absence of ER, PR, and HER2 [4]. Here, ESR1 is linked to breast cancer.