Evolution of the Hox gene cluster.

The Hox genes are a family of developmental control genes containing a homeobox motif, and tend to be organised in distinctive clustered arrays in animals. Organisation within the cluster can relate to how the genes function. Whilst much has been discovered about the Hox gene cluster in traditional model systems of developmental biology, increasing amounts of data from a wider variety of species are illuminating more about the nature of the Hox cluster deep in animal ancestry, as well as revealing the evolutionary flexibility and derivations along present-day lineages. The consensus view of the Hox cluster is that it patterns the anterior–posterior axis of bilaterally symmetrical (bilaterian) animals and exhibits the phenomenon of colinearity. There is, however, much evolutionary change within this system. This diversity in the Hox system is linked to the evolution of animal diver- sity and informs our understanding of the pre-bilaterian origins of the Hox genes themselves.