Human MGMT gene is stably present in all normal tissue cells, and its encoded MGMT protein has the highest activity in human liver, followed by lymph gland and intestinal tract, and the activity of bone marrow cells is the lowest. As a kind of DNA repair proteins, MGMT can remove the alkyl adduct at O6 locus of guanine from DNA, which can cause mutagenicity and cytotoxicity, thereby protecting the cells against the damage of the alkylation group. Therefore the function of MGMT is the main reason of cancer’s resistance to the alkylating agent drugs. Guidelines for Detection Technology of Tumor Individualized Therapy states that only patients with methylation at promoter of MGMT can benefit from temozolomide treatment.