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Nucleotide hybrid residues. (A) Definition of the hybrid residues for the mutation of thymine into guanine and adenine into cytosine. A complete copy of each heterocycle is used for the purine→pyrimidine transition. (B) Hybrid residues for the mutation of guanine into adenine and cytosine into thymine. In this case, the heterocycles are shared and only dummy atoms for the second state are added (yellow).
