Frequency distributions of percent difference in total coding sequence length (exons only) between human and mouse orthologs (light gray) and between human and chimpanzee orthologs (black) for 7645 protein-coding genes. Forty-nine percent of the genes exhibit some length difference between human and mouse; human and chimpanzee orthologs exhibit length difference in only 17 of these 7645 genes. Data from Clark et al. (2003).