with either the agonist diethylstilbestrol (top, ) or antagonist 4-hydroxytamoxifen (bottom, ). The ligands are depicted as space filling spheres (white = carbon, red = oxygen). When an agonist is bound to a nuclear receptor, the C-terminal alpha helix of the LBD (H12; light blue) is positioned such that a coactivator protein (red) can bind to the surface of the LBD. Shown here is just a small part of the coactivator protein, the so-called NR box containing the LXXLL amino acid sequence motif. Antagonists occupy the same ligand binding cavity of the nuclear receptor. However antagonist ligands in addition have a sidechain extension which sterically displaces H12 to occupy roughly the same position in space as coactivators bind. Hence coactivator binding to the LBD is blocked.]] Estrogenic compounds span a spectrum of activity ranging from: Full agonists (agonistic in all tissues) such as the natural endogenous hormone estradiol Mixed agonists/antagonistics (agonistic in some tissues while antagonistic in others) such as tamoxifen (a SERM). Pure antagonists (antagonistic in all tissues) such as fulvestrant. SERMs are known to stimulate estrogenic actions in tissues such as the liver, bone and cardiovascular system but known to block estrogen action where stimulation is not desirable, such as in the breast and the uterus. This agonistic or antagonistic activity causes varied structural changes of the receptors, which results in activation or repression of the estrogen target genes. SERMs interact with receptors by diffusing into cells and there binding to ERα or ERβ subunits, which results in dimerization and structural changes of the receptors. This makes it easier for the SERMs to interact with estrogen response elements which leads to the activation of estrogen-inducible genes and mediating the estrogen effects. SERMs unique feature is their tissue- and cell-selective activity. There is growing evidence to support that SERM activity is mainly determined by selective recruitment of corepressors and coactivators to ER target genes in specific types of tissues and cells. SERMs can impact coactivator protein stability and can also regulate coactivator activity through post-translational modifications such as phosphorylation. Multiple growth signaling pathways, such as HER2, PKC, PI3K and more, are downregulated in response to anti-estrogen treatment. Steroid receptor coactivator 3 (SRC-3) is phosphorylated by activated kinases that also enhance its coactivator activity, affect cell growth and ultimately contribute to drug resistance. The ratio of ERα and ERβ at a target site may be another way SERM activity is