Like those of the fruit fly, vertebrate Hox genes are organized in clusters and expressed along the anteroposterior axis. (a) In the mouse, four complexes of Hox genes, comprising 39 genes in all, are present on four different chromosomes. Not every gene is represented in each complex; some have been lost in the course of evolution. (b) The Hox genes are expressed in distinct domains along the anteroposterior axis of the mouse embryo. The color shading represents the different groups of genes shown in part a.
[S. B. Carroll, “Homeotic Genes and the Evolution of Arthropods and Chordates,” Nature 376, 1995, 479-