Parent genes were designated based on highest homology to the known human protein. Identified gene duplications were sequentially named according to nomenclature guidelines, based on decreasing sequence homology to the parent gene. Duplicated genes were further analysed to determine if they represented potentially new protein-coding genes or non-functional pseudogenes. Pseudogenes were identified according to criteria outlined previously [9] and assigned to the following categories: detritus pseudogenes (those which are fragments missing exons) and reverse-transcriptase events (those which resemble mRNA sequences and lack introns). If data suggested that a duplicated gene was protein coding, it was considered to be a new gene family member and named according to the previously established ALDH nomenclature system [14]. Zebrafish aldh genes were named according to the guidelines set out by the zebrafish nomenclature committee (http://www.zfin.org) [15]. Pseudogenes in rodent (or fish) and non-rodent/non-fish genomes were appended with the suffix 'p' or 'P', respectively, and followed by a number designating multiple pseudogenes for a given gene family within each individual species.
