Distribution of C+G and Folding Energy in Codon-Related Subsequences of mRNA. Codons were identified in intact mRNA sequences and phase-selected for subsequences containing only the first (1_1), second (2_2), third (3_3) codon letters or combinations of these subsequences. Subsequence 1_2 means, for example, that the 3rd codon letters were removed from the original mRNA. The G+C content was counted and the dGs were measured by mfold. Each column represent the mean ± S.E.M of n = 87 determinations. a: FE of intact mRNAs and subsequences, b: G+C content of intact mRNAs and subsequences.