Direct repair of methylated nucleotide bases. (a) When the G residue of a normal G≡C base pair (top) is methylated on O6, the resulting O6-methylguanine base-pairs with thymine (bottom) and thus is highly mutagenic. (b) O6-Methylguanine–DNA methyltransferase transfers the methyl group from the O6-methylguanine onto one of its own Cys residues, and the enzyme is thereby targeted for degradation.