Free folding energies in different codon residues. Free folding energies (FFE) were determined in phase-selected subsequences of 81 different genes. The original nucleic acids contained the intact three-letter codons (1st+2nd+3rd). Subsequences were constructed by periodic removal of one letter from the codon and maintaining the other two (1st+2nd, 1st+3rd, 2nd+3rd) or removing two letters and maintaining only one (1st, 2nd, 3rd). Distinction was made between exons (B and D) and the preceding (-1, A) and following (+1, C) sequences (introns). The dG values were determined by mfold and the FFE was calculated. Each bar represents the mean ± SEM, n = 81.