Several lines of experimentation suggest that endogenous retinoids, metabolites of vitamin A, play a role in the anterior/posterior development of the central body axis and the limbs of vertebrates. High levels of endogenous retinoids have been detected in proximity to these developing axes in a variety of vertebrate fetuses. Teratogenesis studies suggest that both retinoid excess and deficiency are capable of disrupting the development of these axes. Finally, retinoic acid receptors regulate many developmental control genes, including homeobox genes and growth factor genes.