ESR1 and breast cancer: Despite the existence of well-established traditional prognostic markers, such as tumor size, patient age and menopausal status, tumor axillary lymph node involvement, lymphatic/vascular invasion status, hormone receptor status (estrogen (ER) and progesterone (PR)), status of human epidermal growth factor 2 receptor (HER2), and the pathologic prognostic stage (according to the AJCC Cancer Staging Manual, 8th edition [2]), clinical practice shows that, solely on the basis of these markers, it is not possible to identify the prognosis of all patients with BC reliably.