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The two-tailed comet assay(2T-comet assay)is a method for simultaneously evaluating DNA single-strand breaks (SSBs) and double-strand breaks (DSBs). In the present study, the endonuclease DNase I and hydrogen peroxide were used to induce DSBs and SSBs in bone marrow mononuclear cells(BMMNCs)from mice,and the damaged DNAs were assessed with a 2T-comet assay.The results demonstrated that this method can detect and dis-criminate between BMMNC DNA SSBs and DSBs simultaneously. Using this method, we studied DNA damage in BMMNCs from female BALB/c mice after total body irradiation with X-rays and carbon ions. The results indicated that these two types of radiation induced serious DNA damage in BMMNCs in a dose-dependent manner. The DNA damage induced by carbon ions was more severe than that induced by X-rays at the same dose, and a high dose of carbon ion radiation was more likely to cause death in mice.The DSBs and SSBs induced by X-rays were the highest on the 3rd day post-IR. For carbon ion radiation, DSBs were the most serious on the 3rd day, while SSBs were more obvious on the 3rd day and 13th day post-IR.The ratio of DSBs/SSBs was clearly related to the different types of radiation.