ALDH gene-naming conventions dictate that (i) ALDH superfamily members sharing more than ~40 per cent AA identity belong to the same family (eg ALDH1A, ALDH1B, etc.), and (ii) ALDH family members that share greater than 60 per cent AA identity belong to the same subfamily (eg ALDH1A1, ALDH1A2, etc). This provides a convenient and systematic naming system for an entire superfamily. Interestingly, this does not always indicate homology properly; these rules in the cytochrome P450 (CYP) gene superfamily are known to break down when one includes evolutionarily distantly related animals [27]. For example, whereas zebrafish Aldh3d1 and Aldh3b1 share only 50 per cent AA identity, HomoloGene evidence and alignments suggest that aldh3d1 is probably a duplication of aldh3b1 (data not shown). Although aldh3d1 has diverged considerably, it is likely to be more closely related to aldh3b1 than the naming convention would suggest.
