Homeobox genes and congenital malformations.

In this review we have built a case for abnormal Hox gene expression in human congenital malformations without presenting any direct evidence of their involvement. This approach is justified by the dramatic advances in developmental genetics which emphasize the considerable similarity in the primary processes and molecules used to guide early morphogenesis in all species. Hox genes occupy a central role in this scheme, being activated in a specific rostral-caudal order after initial specification of the basic embryonic axes and, thereafter, specifying positional identity by influencing downstream "realizator" genes that carry out the position-specific program. These theoretical arguments, together with the dramatic results obtained in an evolutionarily similar organism (the mouse) using the transgenic and gene deletion approaches, make it highly likely that abnormalities in Hox gene structure and expression will soon be implicated in specific human congenital malformation syndromes. In parallel with this phenotypic analysis, we can expect that the animal models discussed in this review will provide greater detail regarding the upstream regulators and downstream targets of Hox gene products. Together these approaches promise to finally elucidate some of the underlying mechanisms responsible for human congenital malformations.