Identifying HOX paralog groups by the PBX-binding region

The Hox genes are a family of transcription factors that define specific anteroposterior identities, both in vertebrate and in invertebrate embryos, and that are characterized by a very highly conserved DNA-binding motif known as the homeodomain. In vitro, most HOX proteins recognize the same four-base-pair consensus sequence that is actually repeated many times in the genome. Far greater binding specificity is achieved when HOX proteins bind as heterodimers with PBX proteins (vertebrate homologs of Drosophila homeodomain-containing transcription factor extradenticle). PBX and HOX proteins interact at a specific and highly conserved hexapeptide on the surface of the HOX protein. This short sequence of amino acids is necessary for PBX binding and, apart from the homeodomain itself, is the most characteristic feature of Hox genes.