ESR1 and breast carcinoma: In clinical practice, molecular subtypes of breast cancer are classified based on prognostic and predictive markers, including steroid hormone receptor status such as estrogen receptor (ER) and progesterone receptor (PR) and the presence or absence of the human epidermal growth factor receptor 2 (HER2), also known as ErbB-2, ERBB2, or HER2/neu [5].