O6-methylguanine methyltransferase (MGMT) is a ubiquitous DNA repair protein that removes O6 -alkyl-guanine lesions, primarily O6 -methyl-guanine from damaged DNA. It is a major contributor to cellular protection from the mutagenic, carcinogenic, and cytotoxic effects of DNA alkylation. Also, MGMT is frequently silenced due to abnormal methylation during cancer development. Consequently, the loss of MGMT activity that made the cell more likely to become a cancer cell in the first place also makes tumor cells more sensitive to radiation therapy and certain alkylating drugs.