Evidence for repair of premutational damage from split x-ray doses

Chinese hamster ovary cells were given two 450-rad doses of X rays separated by 1 or 2 h, during which the cells were held at room temperature to minimize progression through the cell cycle. The frequency of induced mutations to 6-thioguanine resistance was compared with that of mutations induced by a single dose of 450 or 900 rad. Mutation frequencies induced by the split dose were intermediate between those induced by the two single doses, suggesting that premutational lesions were repaired.