ESR1 and breast cancer: Over the years, many studies have identified prognostic factors in breast cancer, such as age, tumor size, axillary lymph node status, histologic grade, estrogen/progesterone receptor (ER/PR) status, human epidermal growth factor receptor 2 (HER2), and Ki-67, which are significant factors that should be considered when deciding on adjuvant treatments (3, 4).