Retinoic acid, a metabolite of vitamin A, is necessary for normal organogenesis but acts as a teratogen at high levels during embryonic and fetal development. Retinoic acid functions through its interaction with the nuclear protein, retinoic acid receptor (RAR). RAR belongs to the steroid and thyroid hormone superfamily of nuclear receptor proteins which exert their effects by binding to specific DNA response elements, thus regulating gene expression in target cells. RAR exists as three major subtypes: alpha, beta and gamma.