ESR1 and breast carcinoma: ER antagonism, using a selective estrogen receptor modulator (SERM), such as tamoxifen or a selective estrogen receptor degrader (SERD), such as fulvestrant, is yet another form of anti-estrogen therapy that is widely effective in the treatment of breast cancer, providing greater than 10 year survival in postmenopausal women with early stage, ER-positive cancer (102).