Nevertheless, much remains to be learned about positive selection in mammalian genomes, even within protein-coding regions. The most comprehensive scans for PSGs so far [4, 5, 7, 8, 9] have been based on at most three genome sequences-typically the highly similar human, chimpanzee, and/or rhesus macaque genomes (.97% average identity in orthologous coding regions [8] ). As a result, the power for detection of PSGs has been relatively weak [5, 8] . In addition, in several of these studies, at least one genome was of draft quality, which reduced the number of genes that could be examined and required additional care in avoiding false positive predictions.