This heterogeneous disease can be classified into five molecular subtypes, luminal A, luminal B, human epidermal growth factor receptor 2 (HER2)-positive, basal-like, and normal-like BC, according to the expression of the estrogen receptor (ER) progesterone receptor (PR), the overexpression of HER2 cytokeratin CK, and the proliferation index Ki67 (Fig. 1). Here, ESR1 is linked to breast cancer.