Organisation of multicellular animals involves the action of genes that impart “positional information”. All vertebrates are built on a segmental pattern that is most obviously expressed by the appearance of somites during embryonic development.

A common feature of genes that impart individual identity (and therefore positional information) to specific segments is the possession of a “homeobox” DNA binding motif coding for a consensus sequence of 60–63 amino acids that acts as a transcriptional regulator of “downstream” genes. The most widely researched homeobox genes are the so-called homeotic selector genes of the Antp-type (the defining gene is named Antennapaedia). In the fruit fly Drosophila, these are situated on chromosome 3 as part of the HOM cluster. HOM-C genes are strongly conserved during evolution and in mammals have been replicated to appear on separate chromosomes in four paralogous complexes called Hox clusters. They are expressed principally in developing ectodermal and mesodermal tissues and in general terms are responsible for segmental specification of the dermatomes, musculoskeletal, and nervous systems.¹ However, Hox genes are not expressed in the greater part of the gut endoderm but in their place, mammalian members of the Para-Hox genes²—an “evolutionary sister” of the Hox clusters—seem to play an important role in gut patterning. Members of this group are Pdx1 which is required for the correct development of the pancreas and duodenum³ and three homologues of the Drosophila gene Caudal which in mammals are called Cdx1, Cdx2, and Cdx4. In addition to their own unique domains, the Cdx genes exhibit significant topographical overlap of expression during development, as well as in the …