RARS1 and acute promyelocytic leukemia: In contrast, the isomeric RA variant, 9-cis retinoic acid binds to retinoid X receptors (RXRs), which can exist as heterodimers with RARs or other nuclear receptors.1 Retinoids (synthetic ligands that engage RARs) and rexinoids (RXR-binding compounds) have therapeutic value against a variety of metabolic diseases and cancer, particularly for the treatment of promyelocytic leukemia.