The homeotic and cephalic gap genes play central roles in the specification of the anteroposterior animal body axis. Genetic studies carried out in Drosophila and mouse now demonstrate that these genes are also involved in embryonic brain development. The homeotic genes act in posterior brain patterning, and the cephalic gap genes act in anterior brain patterning. Moreover, striking cross-phylum gene replacement experiments show that invertebrate and vertebrate members of the orthodenticle gene family can functionally replace each other. These findings indicate that the genetic mechanisms involved in embryonic brain development are conserved and suggest a common evolutionary origin of the insect and vertebrate brain.