Hox genes have been shown to be important regulators of pattern formation in vertebrates. Retinoic acid has been shown to affect the expression of Hox genes in vitro and in vivo, and some of its effects on development correspond to changes in Hox gene expression. The idea that retinoic acid is not simply a powerful pharmocological agent, but rather that it plays an important role in creating the normal expression patterns of Hox genes, is provided by the recent identification of retinoic acid responsive enhancers near Hox genes.