BC is classified into five major subtypes according to molecular features and intrinsic expression of the genes encoding the estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2): luminal A (ER and/or PR positive and HER2 negative), luminal B (ER or PR positive and HER2 positive), HER2 overexpressing, normal-breast like and basal-like or triple-negative breast cancer (TNBC) phenotype [3]. Here, ESR1 is linked to breast cancer.