ESR1 and breast cancer: Yet, studies on breast cancer prognosis according to SEP did not distinguish pre- from postmenopausal breast cancer [8–11], lacked treatment-specific information [8, 15, 21, 22], failed to stratify by ER status [8–11, 14, 15, 21, 22], and included patients with metastatic breast cancer [11, 13, 14, 22, 23].