Enrichment of MSH2^–/– CFU after TMZ treatment. Lethally irradiated wild-type mice received transplants of a mixture of 10% MSH2^–/– (□) and 90% wild-type (▪) whole bone marrow, followed by treatment 3 weeks later with 80 mg/kg TMZ daily times 3. Bone marrow was harvested 8 weeks after transplantation, and enrichment of MSH2^–/– cells was determined by PCR of individual CFUs. There was a significant increase in the proportion of MSH2^–/– CFUs recovered from treated versus untreated mice; P < .0001 (Fisher exact test).