ParaHox cluster is the paralogue of the Hox cluster which was before the work of Brooke et al. considered to be orphan genes.Brooke et al.5, who found that three of these orphan genes were closely linked in the AMPHIOXUS genome.The para Hox cluster caudal-type homeobox (Cdx), genomic screened homeobox (Gsh) and Xenopus laevis homeobox 8/insulin promoter factor 1 (Xlox/Ipf1) is so far restricted to Chordates but individual genes can be found scattered in other Deuterostomes and protostomes genomes.

Like Hox genes ,para Hox genes also believed to arose before the divergence of Cnidarians and Bilateria by means of duplication of hypothetical proto hox cluster.The origin and original structure of the Hox and ParaHox clusters played a vital role in the the evolutionary changes that accompanied the origin of Bilaterians and the Cambrian Explosion. Cnidarians have an unique distinction of possessing both Hox and para Hox genes for the first time as more basal organisms( Cntenophores and sponges)lack both Hox and para Hox genes.The origin of these clusters in ancestors of Cnidarians had led to huge diversity in body plan during cambrian explosion which also also saw emergence of many bilaterians.

Other players of Antp class:

Hox and para Hox clusters belong to Antp class of proteins so do even-skipped homeotic gene (Evx) and mesenchyme homeobox (Meox; also known as Mox).Phylogentic analysis indicate that these evx and Mox proteins are close to Hox and ParaHox genes, and are located on either side of the Hox cluster in vertebrates.Evx is also found closely to 5′ end of Hox genes in Cnidarians also.Three other members of Antp class namely EGHBox30, made up of of motor neuron restricted (Mnx), gastrulation brain homeobox (Gbx) and engrailed (En).Comparative genmics studies support presence of an extended mega Hox cluster early during metazoan evolution comprising of :
Mnx,Gbx, En, Evx, Hox, Meox and ParaHox genes

Further reading on Para Hox clusters:

Hox, ParaHox, ProtoHox: facts and guesses
J Garcia-Ferna`ndez
Heredity (2005) 94, 145–152

Brooke, N. M., Garcia-Fernàndez, J. & Holland, P. W. H.
The ParaHox gene cluster is an evolutionary sister of the Hox gene cluster.
Nature 392, 920–922 (1998).

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