Age-dependent codon usage for fixed GC content and length. For each of human, mouse, and fly (left to right), its genes were binned by either their GC contents (top) or lengths (bottom). For each such bin, the number of codons (out of the 59 analyzed) with statistically significant age-dependent variance is shown. A particular codon was labeled as being age-dependent if its variance between ages was different than random (false discovery rate (FDR) corrected for multiple hypotheses < 0.05). Red 'X' mark bins for which the sub-division into age groups resulted in some groups having fewer than 5 genes; the results for such bins should be disregarded, since they are statistically inadequate.