Consequently, mutations in Hox genes alter segmental identity and cause morphological defects. In mammals, 39 Hox genes are distributed over four clusters, each containing 9 to 11 genes closely packed in less than 150-kb of sequences. Their spatio-temporal expression profile during embryogenesis reflects their arrangement in the clusters: the 39 most genes are expressed earlier and their expression domain reaches a more anterior limit than those occupying 59 positions. As a Levodopa result, members of the Hox complexes are expressed in nested and overlapping domains along the Tetrabenazine developing body suggesting that specific combinations of HOX proteins provide a unique address to defined regions. Based on sequence homology and location within clusters, Hox genes are also classified into 13 paralog groups. The Hox clustered organization is fundamental for the precise regulation and the function of each gene and hence for the correct formation of the embryo. Analysis of Hox mutant mice endorses the collinear relationship between the position of individual genes within Hox clusters and the structural defects observed along the anterior-posterior axis. For example, the mutation of the Hoxa5 gene, located in the middle of the HoxA multigenic complex, affects axial specification at the cervico-thoracic level. A high percentage of Hoxa5-/- pups die at birth from impaired respiratory tract development. Moreover, the loss of Hoxa5 function results in panoply of phenotypes indicative of the broad range of Hoxa5 actions throughout life. Most defects in Hoxa5-/- mutants are confined to the cervico-thoracic region corresponding to the Hoxa5 rostral-most expression domain, where the major Hoxa5 transcript of 1.8-kb encoding the HOXA5 270-amino-acid protein, is specifically expressed. Thus, Hoxa5 appears as a critical determinant in the specification and the development of a subset of structures at the cervico-thoracic level. While the developmental role of Hox genes is well established, the regulation of Hox gene expression in the embryo remains incompletely understood.