Oxidative damage to purine and pyrimidine bases. (a) Common positions (circled in red) of oxidative damage to nucleotide bases (left) and the products of oxidation at these positions (right). (b) Oxidation of the guanine of a G residue to 8-oxoguanine is extremely mutagenic. The 8-oxoG residue base-pairs with either A or C and thus can result in a G≡C to T=A transversion.