CYP19A1 and breast cancer: In large randomized preventive trials of tamoxifen and the aromatase inhibitor anastrozole, these endocrine therapies have reduced the risk of ER-positive breast cancer by 30%-50%, particularly in women with an increased risk of breast cancer.69–74 However, tamoxifen did not reduce breast cancer risk in women who never used estrogen-alone HRT (RR 1.00, 95% CI 0.67–1.50).71 Counterintuitively, the tamoxifen-mediated risk reduction of breast cancer was pronounced among women who also used estrogen-alone HRT (RR 0.43, 95% CI 0.2–0.95).71