Extensive in situ hybridization analysis has been performed on the mouse HOX genes, allowing detailed comparison to similar data from the related Drosophila homeotic genes. Mouse HOX genes have region-specific patterns of expression during embryogenesis. Each gene is expressed within a characteristic antero-posterior region of the early embryo, and later in the central nervous system. Many of the genes are also expressed within similar, but non-coincident, antero-posterior limits in the peripheral nervous system (for example cranial and dorsal root ganglia), somitic mesoderm and visceral organs. These expression patterns suggest that vertebrate HOX genes, like Drosophila homeotic genes, are involved in the specification of cell fate according to position along the antero-posterior axis. Apart from antero-posterior body patterning Hox genes show specific expression patterns in number of developing rudiments, such as limb buds, developing head structures, etc.