Contour plot of the average FR vs. CDR mutability over sets of artificial variants of human V_H sequences. We constructed (A) 10⁴ sets with identical nucleotide composition (black), 10⁴ sets with identical codon composition (blue), and (B) 10⁴ sets with identical amino acid translation (black). The contour levels are drawn at 1, 10, and 100 sequences. The actual sequence set is shown in red.