MGMT and cancer: Tumor resistance manifests because MGMT successfully repairs the O6-methylguanine and O6-chloroethylguanine lesions induced by methylating agents [temozolomide (TMZ), procarbazine] and chloroethylating agents [1,3-bis-2-chloroethylnitrosourea (BCNU) and CCNU], respectively; the repair action prevents the mutagenic effects and formation of cytotoxic interstrand DNA cross-links in cancer tissues.