Specificity
Two families of retinoid receptors, RARs and RXRs, have been identified. Retinoic acid receptors (RARs) include RARa, RARb and RARg, each of which have a high affinity for all trans-retinoic acids and belong 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, RXRa, RXRb and RXRg, are activated by 9-cis-RA, a stereo- and photo-isomer of all trans-RA that is expressed in vivo in both liver and kidney and may represent a widely used hormone. As is true for the RAR subfamily, the RXR receptors are closely related to each other both in their DNA-binding and ligand-binding domains and are encoded by separate genes at distinct chromosomal loci.
