These drugs have been used for the treatment of estrogen receptor–positive breast cancers and have been shown to decrease estrogen’s effects at the level of the breast tissue.13 Tamoxifen is used in both the treatment and the prevention of estrogen receptor–positive breast cancer and is a common therapy for premenopausal women and an option for postmenopausal women.12 This evidence concerns the gene ESR1 and breast carcinoma.