An abundant oxidative lesion, 8-oxo-7,8-dihydroguanine (8-oxoG), often directs the misincorporation of dAMP during replication. To prevent muta-tions, cells possess an enzymatic system for the removal of 8-oxoG. A key element of this system is 8-oxoguanine-DNA glycosylase (Fpg in bacteria, OGG1 in eukaryotes), which must excise 8-oxoG from 8-oxoG:C pairs but not from 8-oxoG:A.