Evolutionary relationships among the species included in this work. The divergence date between the lineage leading to Chondrichthyes (to which Heterodontus, the horn shark, belongs) and that leading to the clade of all other taxa on this tree is about 500 millions years. Actinopterygii (the ray-finned fishes) and Sarcopterygii (the tetrapods) diverged about 450 million years ago. Teleosts radiated more than 200 million years ago. The divergence between human and mouse is dated to about 80 million years (Pough et al. 1999). Horn shark, mouse, and human have a single HoxA cluster, while all fishes examined so far have two (see text for details). Among fishes, independent gene losses took place in zebrafish and pufferfish relative to tilapia. Solid boxes represent individual genes. Duplicated clusters are designated as α or β. Pseudogenes are marked with a cross. Question marks represent genomic regions that are not yet characterized.