The retinoic acid receptor alpha (RAR alpha) is a nuclear hormone receptor and can function as a ligand inducible transcription factor capable of acting as a co-repressor and/or co-activator for gene expression. Nuclear receptors contain a series of conserved domains or regions. These domains/regions include a variable NH2-domain (A/B region), a conserved DNA-binding domain (DBD or region C), a linker region (region D), a ligand binding domain (LBD or region E), and in some receptors a variable COOH-terminal (region F). The RAR DNA binding domain recognizes and interacts with the retinoic acid receptor response elements (RAREs). RAR binds as a heterodimer with RXR. The RAR/RXR heterodimer is associated with corepressor proteins at the RAREs in the absence of ligand. Upon binding of ligand, RAR undergoes a conformational change, releasing corepressor proteins and allowing for the interaction with coactivator proteins that enhance RARE driven gene transcription. RAR alpha is a member of the Retinoic Acid Receptor family. Retinoids, vitamin A and its natural and synthetic analogues, are a very important group of hormones that regulate a wide variety of biological functions including embryogenesis, cell growth, and cell differentiation. Cotransfection experiments showed that RARs are activated by either all-trans- or 9-cis-retinoic acid at a ligand concentration of 5 X 10^-8 mmol/L.