Human 8-oxoguanine-DNA glycosylase (OGG1) efficiently removes muta-genic 8-oxo-7,8-dihydroguanine (8-oxoGua) and 2,6-diamino-4-hydroxy-5-formamidopyrimidine when paired with cytosine in oxidatively damaged DNA. Excision of 8-oxoGua mispaired with adenine may lead to GfiT transversions.