Fraction of conserved amino acids for which there is variation in nucleotide sequence across strains (that is, alternative codons used for the same amino acid in a give location in the proteome). The amino acids are listed along the y axis, sorted by the number of codon options for each amino acid (indicated by the number immediately preceding the one-letter amino acid abbreviation). (A) The less- to moderately divergent species, and (B) the moderately to more-divergent species (Marburg virus, JC virus, and maize streak virus could have been included in either plot).