Phylogenetic distribution of the Otp - Rx - Hbn cluster. Several sequenced metazoan genomes were searched for evidence of linkage between orthopedia, rx and homeobrain. The genomic organization of the fruit fly homeobrain related gene was previously reported identified by Walldorf et al. . Predicted linkage of these three genes was determined for insects (mosquito, honeybee, flour beetle), chordates (vertebrates, amphioxus, ascidian), sea urchin, two lophotrochozoans (limpet, polychaete annelid), sea anemone and placozoan by querying the available genome sequences using the annotated/predicted gene sequences for each gene (this study). Solid lines between genes represent linkage on a single chromosome, scaffold or contig. Approximate distance (kb between genes) is noted above each line, and the relative transcriptional orientation of each gene is indicated by the direction of the arrow (the arrow points from 5' to 3'). Question marks indicate uncertain linkage. Genes confirmed to be on different chromosomes or genomic scaffolds are represented by squares with no interconnecting lines.