Abstract: It is very common phenomenon that mismatch repair(MMR)deficiency in various tumors, both stem cells and proliferation cells.With MMR deficiency, the cells treated by chemotherapy drugs passed the G₂/S arrest and became resistant to the drugs.In the clinical trial, even in the glioma with negative O⁶-methylguanine-DNA methyltransferase expression show the resistance to Temozolomide.The MMR deficiency cells treated by radiotherapy show two contradictory behaviors.MMR deficiency enhanced the cells resistance to radiation, for cells show less apoptosis or autophagy.On the other hand, pretreated with radiation enhancer, such as 5-iodo-2'-deoxyuridine(IUdR), resulting in more IUdR-DNA unrepaired cells and increasing the sensitivity to radiation.