Retinoids are metabolites of vitamin A (retinal) and are believed to represent important signaling molecules during vertebrate development and tissue differentiation. Two families of retinoid receptors have been identified. Retinoic acid receptors (RARs) include RAR alpha, RAR Beta and RAR gamma, each of which has a high affinity for all trans retinoic acids and belongs to the same class of nuclear transcription factors as thyroid hormone receptors, vitamin D3 receptor and ecdysone receptor. The ligand binding domains of the RARs are highly conserved and RAR isoforms are expressed in distinct patterns throughout development and in the mature organism. Members of the retinoid X receptor (RXR) family, RXR alpha, RXR Beta and RXR gamma, are activated by 9 cis retinoic acid, a stereo and photoisomer of all trans RA that is expressed in vivo in both liver and kidney and may represent a widely used hormone.