“…A ll-trans-retinoic acid 1, its isomer 9-cis-retinoic acid 2, and other natural retinoids are important signaling molecules that act in the embryo 1 and throughout adult life. 2−4 At the genomic level, retinoids bind the retinoid receptors (RARs, subtypes RARα,β,γ) 5 and retinoic X receptors (RXRs, subtypes RXRα,β,γ) 6 to regulate many important biological activities.…”