The blood of 18 patients with acute leukemia was exposed to UV light. Within 2 hours, in all the patients (except one under chemotherapy), nearly 100 per cent of the leukemic blasts showed uptake of 3HTdR as a result of DNA repair replication. Similar exposures of CML blood white cells and chicken blood red cells to UV indicated that myeloid and erythroid cells have no capacity to repair DNA after they have stopped dividing as a result of maturation. This suggests that nonproliferating leukemia blasts are resting cells rather than "end cells."

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