The retinoic acid receptor (RAR) is a type of nuclear receptor which can also act as a transcription factor[1] that is activated by both all-trans retinoic acid and 9-cis retinoic acid.[2] There are three retinoic acid receptors (RAR), RAR-alpha, RAR-beta, and RAR-gamma, encoded by the RARA, RARB, RARG genes, respectively. Each receptor isoform has ten splice variants: four for alpha, four for beta, and two for gamma. As with other type II nuclear receptors, RAR heterodimerizes with RXR and in the absence of ligand, the RAR/RXR dimer binds to hormone response elements known as retinoic acid response elements (RAREs) complexed with corepressor protein. Binding of agonist ligands to RAR results in dissociation of corepressor and recruitment of coactivator protein that, in turn, promotes transcription of the downstream target gene into mRNA and eventually protein. In addition, the expression of RAR genes is under epigenetic regulation by promoter methylation.[3]
