| Summary: | The <em>Pdx1</em> or <em>Ipf1</em> gene encodes an important homeodomain-containing protein with key roles in pancreas development and function. Mutations in human <em>PDX1</em> are implicated in developmental defects and disease of the pancreas. Extensive research, including genome sequencing, has indicated that <em>Pdx1</em> is the only member of its gene family in mammals, birds, amphibians, and ray-finned fish, and with the exception of teleost fish, this gene forms part of the ParaHox gene cluster along with <em>Gsx1</em> and <em>Cdx2</em>. The ParaHox cluster, however, is a remnant of a 4-fold genome duplication; the three other ParaHox paralogues lack a <em>Pdx</em>-like gene in all vertebrate genomes examined to date. We have used bacterial artificial chromosome cloning and synteny analysis to show that the ancestor of living jawed vertebrates in fact had more ParaHox genes, including two <em>Pdx</em> genes (<em>Pdx1</em> and <em>Pdx2</em>). Surprisingly, the two <em>Pdx</em> genes have been retained in parallel in two quite distantly related lineages, the cartilaginous fish (sharks, skates, and chimeras) and the Indonesian coelacanth, Latimeria menadoensis. The <em>Pdx2</em> gene has been lost independently in ray-finned fish and in tetrapods.
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