THE FUNCTION AND EVOLUTION OF MSX GENES - POINTERS AND PARADOXES

The Msx genes of vertebrates comprise a small family of chromosomally unlinked homeobox-containing genes related to the Drosophila gene musc le-segment homeobox (msh). Despite their ancient pedigree, the Msx gen es are expressed in a range of vertebrate-specific tissues, including neural crest, cranial sensory placodes, bone and teeth. They are activ e in numerous systems, which have been used as models to study pattern formation and tissue interaction, and are, therefore, attracting a gr owing interest among developmental biologists. But beyond their presum ed role as transcription factors, we do not know what their functions are in the cell or the embryo. Here, I review recent evidence that is beginning to address this problem and might eventually increase our un derstanding of bow the vertebrate embryo has evolved.