O6-Methylguanine-DNA Methyltransferase (MGMT) is an important DNA repair protein involved in tumor cell resistance to the cytostatic activity of chemotherapeutic alkylating agents. This protein is also effective in protecting normal cells against the genotoxic and carcinogenic effects of DNA alkylation. The alkylating drug resistance is caused by MGMT's ability to remove DNA alkyl groups introduced in the O6 position of guanine. MGMT is expressed in highly variable amounts, depending upon the cell and tissue type, species, and cellular growth characteristics. In addition, MGMT activity varies among groups of tumors and within a particular type of tumor.