Propensity for PC-compatible codon pairs. There are 4096 possible codon pairs altogether (64 × 64, including those formed with the 3 stop codons). Codons that are (P) or are not (N) coded by a specific complementary rule (PC) were counted. The bars represent the mean ± SD of 9 independent determinations (n = 9). PC1 and PC2 indicate complementarity of all 3 or only the first and third codon bases in parallel (C) or anti-parallel (RC) readings.