The mouse Engrailed, Wnt and Pax genes, which are homologues of Drosophila segmentation genes, have provided a critical genetic entry point for dissecting the molecular and cellular control of mesencephalon and metencephalon development in vertebrates. Mutant phenotypes and gene expression data suggest that six members of these gene families are required for early formation of these brain regions. Ectopic transplantation studies have shown that the midbrain-hindbrain-junction protein can act as an organizer and recruit certain host cells to re-establish parts of the entire region. Taken together, these studies indicate that the mesencephalon and metencephalon develop as one independent unit, and that the genetic network regulating development of this region involves conserved genes that control segmentation in Drosophila. By analogy, segmentation of the rest of the brain might best be described in terms of 'genetic units' defined by genetic and transplantation data.