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Alkylating agents are frequently used in the chemotherapy of many types of cancer. This group of drugs mediates cell death by damaging DNA and therefore, understandably, cellular DNA repair mechanisms can influence both their antitumour efficacy and their dose-limiting toxicities.This review focuses on the mechanism of action of the DNA repair protein, O6-methylguanine-DNA methyltransferase (MGMT) and its exploitation in cancer therapy and reviews the current literature.MGMT can provide resistance to alkylating agents by DNA damage reversal. Inhibition of tumour MGMT by pseudosubstrates to overcome tumour resistance is under clinical evaluation. In addition, MGMT overexpression in haematopoietic stem cells has been shown in animal models to protect normal cells against the myelosuppressive effects of chemotherapy: this strategy has also entered clinical trials.MGMT inhibitors enhance the myelotoxic effect of O6-alkylating drugs and therefore reduce the maximum-tolerated dose of these agents. Retroviral vectors used for chemoprotective gene therapy are associated with insertional mutagenesis and leukaemia development.The results of ongoing preclinical and clinical research involving various aspects of MGMT modulation should provide new prospects for the treatment of glioma, melanoma and other cancer types.Tissue- and tumour-specific approaches to the modulation of MGMT together with other DNA repair functions and in combination with immuno- or radiotherapy are promising strategies to improve alkylating agent therapy.