Specification of jaw subdivisions by Dix genes

The success of vertebrates was due in part to the acquisition and modi cation of jaws. Jaws are principally derived from the branchial arches, embryonic structures that exhibit proximodistal polarity. To investigate the mechanisms that specify the identity of skeletal elements within the arches, we examined mice lacking expression of Dlx5 and Dlx6, linked homeobox genes expressed distally but not proximally within the arches. Dlx5/6(-/-) mutants exhibit homeotic transformation of lower jaws to upper jaws. We suggest that nested Dlx expression in the arches patterns their proximodistal axes. Evolutionary acquisition and subsequent refinement of jaws may have been dependent on modi cation of Dlx expression.