There are two homologous nuclear receptors for the hormone 17β-estradiol (estrogen), commonly called ERα and ERβ. Receptor activation leads to the formation of homo- and hetero-dimers, which in turn interact with accessory proteins to regulate gene transcription. Tamoxifen is a selective estrogen receptor modulator, evoking tissue-dependent effects. It is an antagonist of ER action in breast tissue and breast cancer cells and is reported to be effective in the treatment of early breast cancer to prevent tumor growth. Importantly, tamoxifen has been reported to act as an ER agonist in bone and blood vessels, helping to minimize osteoporosis and reduce the risk of cardiovascular disease in post-menopausal women. Also, tamoxifen is a partial ER agonist in uterine tissues and is reported to increase the risk of endometrial carcinoma.
