Oxidative degradation of guanine to 2,6-diamino-4-oxo-5-formamidopyrimidine (FapyG) is believed to be mutagenic. It has been proposed recently that the enol tautomer of FapyG is mainly responsible for this effect leading to a guanine-to-thymine mutation (T. H. Gehrke, U. Lischke, K. L. Gasteiger, S. Schneider, S. Arnold, H. C. Muller, D. S. Stephenson, H. Zipse, T. Carell, Nat. Chem. Biol. 2013, 9, 455–461). Here, density functional methods suggest that the enol tautomer of FapyG might not be responsible for the proposed guanine-to-thymine mutation. Instead, it might result in a guanine-to-adenine mutation.