ESR1 and breast carcinoma: Breast cancer is a heterogeneous group of diseases that differ in terms of behavior, prognosis and response to treatment.1, 2 Traditional prognostic and predictive markers, such as tumor size, lymph-node involvement, vascular invasion, and expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) are used to select the treatment.