Phylogenetic tree showing divergence of the hedgehog family members in model organisms in which the genes have been completely sequenced. Mm = mouse, Me = tammar wallaby, Tt = dolphin, Gg = gorilla, Hs = human, Xt = Xenopus. Complete node labels and distances can be found in additional file 8. Each hedgehog subtype (IHH, SHH, DHH) forms a separate lineage. The fish DHH orthologues form a separate cluster from the mammalian DHH genes and have been renamed the fishy-hedgehog (FHH) cluster.