Instability of repetitive sequences is a hallmark of human cancer, and its enhancement has been linked to oxidative stress. Malondialdehyde is an endogenous product of oxidative stress that reacts with guanine to form the exocyclic adduct, pyrimido[1,2- alpha]purin-10(3H)-one (M1G). We used site-specifically modified single- and double-stranded vectors to investigate the mutagenic potential of M1G in bacteria and mammalian cells. M1G induced frameshift mutations (-1 and -2) when positioned in a reiterated (CpG)4 sequence but not when positioned in a nonreiterated sequence in Escherichia coli and in COS-7 cells. The frequency of frameshift mutations was highest when M1G was placed at the third G in the sequence. M1G induced base pair substitutions at comparable frequencies in both sequence contexts in COS-7 cells. These studies indicate that M1G, an endogenously generated product of oxidative stress, induces sequence-dependent frameshift mutations and base pair substitutions in bacteria and in mammalian cells. This finding suggests a potential role for the M1G lesion in the induction of mutations commonly associated with human diseases.