Two families of retinoid receptors, RARs and RXRs, have been identified. Retinoic acid receptors (RARs) include RARα, RARβ and RARγ, 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. Retinoid X receptor α (RXRα) is the founding RXR family member and is predominantly expressed in the liver, kidney, epidermis, intestine, and a variety of tissues. The RXRA/PPARA heterodimer is required for PPARA transcriptional activity on fatty acid oxidation genes such as ACOX1 and the cytochrome P450 system genes.
