ESR1 and breast cancer: In the Preoperative Anastrozole Compared with Tamoxifen (PROACT) trial, 12 weeks of neoadjuvant anastrozole and tamoxifen yielded similar response rates by calliper (50 vs 46.2%; OR 1.24, 95% CI 0.84–1.83, p = 0.29) and ultrasound measures (39.5 vs 35.4%, respectively; OR 1.19, 95% CI 0.82–1.72, p = 0.37) in 451 patients with ER+ large operable or potentially operable breast cancer.