Abstract
Numerous crystal structures of nuclear receptor ligand binding domains (LBDs) are known. The retinoic acid (RAR) and estrogen (ER) receptors are the two members for which a large set of agonists and antagonist complexes are available. Their analysis reveals key features of the RAR and ER ligand binding pocket (LBP) responsible for ligand selectivity. The RAR LBD exhibits a rigid architecture to which the ligand has to adapt, whereas the ER LBD can accomodate numerous ligands of variable shapes.