Retinoids (an analogue of Vitamin-A (all-trans-Retinol)) and Retinoic Acid (a metabolite of Vitamin-A), regulate expression of target genes through binding and activation of the nuclear receptors like RARs (Retinoic Acid Receptors) and the RXRs (Retinoid X Receptors). all-trans-Retinoic Acid, the Carboxylic Acid form of Vitamin-A is of biological significance since it is predominant under most physiological situations. Biologically active ligands for the RARs and RXRs include all-trans-Retinoic Acid, 9-cis-Retinoic Acid, Retinoids among others, yet circulating levels of 9-cis-Retinoic Acid are much lower than those of all-trans-Retinoic Acid and the physiological significance of the isomerization all-trans-Retinoic Acid to 9-cis-Retinoic Acid and vice versa is yet to be ascertained (Ref.1 & 2). The RARs are expressed at higher levels than the RXRs. [...]