Genomics of the HOX gene cluster.

The Hox family of homeobox genes encode transcription factors that control different aspects of metazoan development. They appear clustered in the genomes of those animals in which their relative positions have been mapped. Although clustering is assumed to be a general property of Hox genes in all bilaterians, just a few species have been studied in sufficient detail to support this claim. Linear duplication of genes inside the cluster, as well as full-cluster duplications account for the actual complexity of HOX clusters in the different animal groups that have been studied (mainly vertebrates). Understanding how the Hox genes are regulated during development will depend, ultimately, on the generation of more powerful tools for cloning intact HOX clusters and for elucidating their cis-regulatory components. To clarify the roles of the Hox genes themselves, we will need to characterize in detail their downstream targets, and some progress in this direction is coming mainly from the recent use of arrayed libraries. Moreover, a comprehensive study of Hox target genes in tissues and organisms promises, in the long term, to give us a clear idea of the role that Hox genes play during development and how they have evolved over time.