MGMT belongs to the family of transferases, specifically those transferring one-carbon group methyltransferases. MGMT involved in the cellular defense against the biological effects of O6-methylguanine in DNA. Repairs alkylated guanine in DNA by stoichiometrically transferring the alkyl group at the O-6 position to a cysteine residue in the enzyme. MGMT catalyzes the chemical reaction: DNA (containing 6-O-methylguanine) and proteinL-cysteine into DNA (without 6-O-methylguanine) and protein S-methyl-L-cysteine.