Codon age-responsiveness for the 59 degenerately coding codons. For each codon, the age-dependent variance was calculated. For each genome, the 59 resulting variance scores were rank-ordered. The 59 codons are sorted by their rank-ordering in the human genome, and the rank-ordering in the mouse genome is compared. A strong overall similarity of codon rank-ordering between the human and mouse genomes is shown. Spearman's rank correlation test: ρ = 0.92, p-value < 10-6. The codons are colored according to a standard biochemical grouping of the amino acids for which they encode.