However, within a few weeks, desensitization occurs, and the ovarian estrogen production is inhibited, resulting in postmenopausal levels of circulating estrogens.47,48 On the other hand, tamoxifen acts as a mixed agonist/antagonist that when bound to the breast cancer ER alters its transcriptional activity and expression of proliferative-associated genes. The gene discussed is ESR1; the disease is breast cancer.