Hox genes and the identity of motor neurons in the hindbrain

The Hox genes comprise a gene family of thirty-eight members in vertebrates. The genes are organized in four gene clusters and activated during gastrulation. Analysis of embryonic expression patterns has indicated that morphological segments along the body axis are characterized by different combinations of expressed Hox genes, the 'Hox codes'. Hox codes can be influenced by exposing embryos to retinoic acid, concomitantly with perturbation of segmental identities. In the murine hindbrain this can be clearly demonstrated for the preotic region containing the trigeminal and facial motor nuclei. The correlation between normal or perturbed segment morphology and the Hox codes is reviewed and discussed.