ESR1 and breast cancer: Breast cancer is a heterogeneous disease that is biologically characterized by the expression of one or a combination of steroid hormone receptors, such as estrogen receptor (ER), progesterone receptor (PR), oncogene-epidermal growth factor receptor ErbB2 (Her2neu), or absence of expression of ER, PR, and Her2neu receptors (Triple Negative Breast Cancer (TNBC)) [2].