O-6-methylguanine-DNA methyltransferase (MGMT) is an enzyme that repairsO-6-methylguanine, amutagenic DNA base damaged by endogenous and environmental alkylating agents and is involved in the cellular defense against the biological effects of O-6-methylguanine in DNA. MGMT repairs alkylated guanine in DNA by stoichiometrically transferring the alkyl group at the O-6 position to a cysteine residue in the enzyme.