ESR1 and breast cancer: Risk of recurrence in early stage breast cancer varies widely from patient to patient, and estimation of risk has historically relied on a combination of clinical and pathologic factors, such as tumor grade, size, stage, lymph node involvement, estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2) status, and patient (age, menopausal status) characteristics.