Illustrations for quantifying theoretical compositions. Quantification of theoretical compositions of nucleotide, codon, and amino acid is based on GC (S) and purine (R) contents, which are readily observed from input coding sequences. Model 1 (red) and Model 2 (blue) differ only in how position-dependent GC (S
) and purine (R
) contents are calculated, where i represents codon position (i = 1, 2, 3).