Therapies using selective ER modulators use compounds that act as ER antagonists or agonists based on the tissue specificity.7 The ER antagonists such as ICI 164,384 and Tamoxifen (TAM) can bind to the ER and hinder E2 binding, thus blocking these receptor-mediated effects and inhibiting breast cancer cell proliferation.7 This evidence concerns the gene ESR1 and breast carcinoma.