The repair of DNA containing O6-alkylated guanine is carried out by DNA-[protein]-cysteine S-methyltransferase (), also known as 6-O-methylguanine-DNA methyltransferase. The major mutagenic and carcinogenic effect of methylating agents in DNA is the formation of O6-alkylguanine. The alkyl group at the O-6 position is transferred to a cysteine residue in the enzyme []. This is a suicide reaction since the enzyme is irreversibly inactivated and the methylated protein accumulates as a dead-end product [].
