ESR1 and breast cancer: The benefits from postmenopausal subgroup analysis could be at least partially explained by other studies which documented that postmenopausal breast cancer patients who are ER+ would get most benefit from receiving tamoxifen as adjuvant treatment compared to those in premenopause because the higher endogenous estrogen level might limit the tamoxifen efficacy in premenopausal patients via competitive binding to ER.