Summary of bilaterian ParaHox gene organisation. Boxes indicate ParaHox genes. Boxes on the same line represent genes linked on the same chromosome. Cluster breaks with intervening non-homeobox genes are denoted by double-parallel lines. Gene losses are denoted by crosses. Question marks indicate no information available for the chromosomal location. However, it should be noted that there are flanking non-homeobox genes on both sides of all the ParaHox orthologues with question marks, except for one side of both Gsx and Xlox in sea urchin Strongylocentrotus purpuratus, in which these two ParaHox genes are located at the ends of their respective scaffolds.