ESR1 and breast cancer: Worldwide, breast cancer is the most common cancer in women.1,2 The National Comprehensive Cancer Network Guidelines suggest the use of tamoxifen for 5 years as adjuvant therapy for premenopausal women with estrogen receptor–positive invasive breast cancer or ductal carcinoma in situ.3 Women who remain premenopausal after the first course can consider tamoxifen for another 5 years, whereas postmenopausal women can consider tamoxifen for 5 years or switch to an aromatase inhibitor.3 Although tamoxifen can block the effects of estrogen in the breast tissue, it acts like estrogen in the uterus.