ESR1 and breast cancer: Two meta-analyses on premenopausal women with ER+ breast cancer showed an inverse association between BMI and ER-positivity [6, 7]; obesity was more frequent in patients with ER−/PR− than with ER+/PR+ tumors (OR, 1.49; 95%CI, 1.29–1.73; p = 1 × 10−7).