The first homeobox gene was molecular cloned nearly two decades ago, and since that time tremendous progress has been made in our understanding of the distribution of homeobox genes in the genomes of many animal species and the common functional role the encoded homeodomains play in cell-type specification, morphogenesis and development.

The amino acid sequence of the homeodomain, as well as the presence of other conserved protein domains, has allowed the classification of homeodomain-containing proteins (homeoproteins) into over thirty separate families (e.g. Hox, Dlx, Msx, Otx,...