Retinoic acid is a metabolite of vitamin A that acts as a ligand for both the retinoic acid receptor (RAR) and the retinoid X receptor (RXR). RAR and RXR heterodimerize and act as ligand-dependent transcriptional regulators, with roles in development, reproduction, immunity, organogenesis, and cancer. Retinoic acid primes embryonic stem cells to become neurons.