[2] Mouse has a homologous ParaHox gene cluster on chromosome 5.
The other three human ParaHox genes are remnants from duplicated ParaHox gene clusters that were generated in the 2R genome duplications at the base of vertebrate evolution.
[3] Some vertebrates, notably chondrichthyan fish and coelacanths, have retained an additional ParaHox gene (PDX2).
[4] The ParaHox gene cluster has been proposed to be a paralogue, or evolutionary sister, of the Hox gene cluster;[1] the two gene clusters being descendent from a segmental duplication early in animal evolution, preceding the divergence of cnidarians and bilaterian animals.
[6][7] Gene expression and functional data lends tentative support to this hypothesis,[7][8] although in many animals the roles of the genes have changed in evolution, notably the Gsx gene family which plays a role in brain (not foregut) development in vertebrates.