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Summary
The pulsed field gel elctrophoresis (PFGE) assay was calibrated for the measurement of X-ray-induced DNA double-strand breaks in Chinese hamster ovary (CHO) cells. Calibration was conducted by incorporating [125I]deoxyuridine into DNA, which induces one double-strand break for every disintegration that occurs in frozen cells. Based on the percentage of the DNA migrating into the gel, the number of breaks/dalton/Gy was estimated to be (9·3 ± 1·0) × 10−12. This value is close to (10 to 12) × 10−12 determined by neutral filter elution using similar cell lysis procedures at 24°C and at pH 8·0. Also, the estimate is in good agreement with the value of (11·7 ± 2) × 10−12 breaks/dalton/Gy as measured in Ehrlich ascites tumour cells using the neutral sucrose gradient method (Blöcher 1988), and (6 to 9) × 10−12 breaks/dalton/Gy as measured in mouse L and Chinese hamster V79 cells using neutral filter elution (Radford and Hodgson 1985).