Nuclear receptors are ligand-inducible transcription factors that specifically regulate the expression of target genes involved in metabolism, development, and reproduction. Their primary function is to mediate the transcriptional response in target cells to hormones such as the sex steroids (progestins, estrogens, and androgens), adrenal steroids (glucocorticoids and mineralocorticoids), vitamin D3, and thyroid and retinoid (9-cis and all-trans) hormones, in addition to a variety of other metabolic ligands. More than 100 nuclear receptors are known to exist and, together, these proteins comprise the single largest family of metazoan transcription factors, the nuclear receptor superfamily. Their natural ligands, as well as synthetic ligands (selective receptor modulators, or SRMs), are known to influence the interaction of these receptors with accessory molecules called coregulators.