ESR1 and breast cancer: Breast cancer is a heterogeneous group of tumors subdivided into luminal, HER2, and Triple Negative Breast Cancer (TNBC) types depending on the expression of the estrogen receptor, progesterone receptor, and epidermal growth factor receptor 2 (HER2) (Bareche et al., 2018; Cancer Genome Atlas Network, 2012; Perou et al., 2000).