Alteration of Hox gene expression in the branchial region of the head causes homeotic transformations, hindbrain segmentation defects and atavistic changes

A functional role of Hox genes in patterning the branchial region of the head was predicted on the basis of their expression patterns in rhombomeres, rhombencephalic neural crest cells and pharyngeal arch mesenchyme. The phenotypic consequences of the generation of both gain- and loss-of function Hox gene mutations not only strongly support this prediction, but also provide unexpected insights into the evolution of vertebrate body plan organization.