Retinoids are metabolites of vitamin A (retinol) 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 RARalpha, RARbeta and RARgamma , 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. Two cofactors that function to repress transcription, designated SMRT and N-CoR, have been shown to associate with the thyroid receptor and RAR in their unliganded state and are released from them upon ligand binding. The carboxy termini of both proteins contain receptor interacting domains while their amino termini contain two previously undescribed repressor domains. SMRT (silencing mediator for RARs and TRs) is 1495 amino acids in length with an estimated molecular weight of 168 kDa. N-CoR (nuclear receptor corepressor) is a protein 2453 amino acids in length with a relative molecular weight of 270 kDa.