The repair of DNA containing O6-alkylated guanine is carried out by DNA-[protein]-cysteine S-methyltransferase (<db_xref db="EC" dbkey="2.1.1.63"/>), 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 [<cite idref="PUB00000053"/>]. This is a suicide reaction since the enzyme is irreversibly inactivated and the methylated protein accumulates as a dead-end product. Methylated-DNA--protein-cysteine methyltransferase