Figure 1: Chemical structure of selected physiological and synthetic retinoids. RA exists as several isomers in vivo including all-trans, 13-cis and 9-cis RA (A). The physiological isomers as well as the synthetic retinoids (B) have a cyclic end group, a conjugated side chain, and a polar end group, with the exception of polyprenoic acid (acyclic retinoid) which maintains a closed circular shape but is not cyclic. The synthetic rexinoids (C) vary in their structure but exist primarily in 9-cis conformation. The 9-cis configuration (including physiological 9-cis RA) supports preferential binding to RXRs.