[Radioresistant DNA synthesis in the fibroblasts of a Down's syndrome patient].

DNA synthesis after gamma-irradiation was analysed either by direct assay of the amount of 3H-Td incorporated into DNA of fibroblasts derived from normal donor and from a patient with Down's syndrome, or by analysis of the steady-state distribution of 3H-DNA in alkaline sucrose gradients. Doses of gamma-radiation that markedly inhibited the rate of DNA synthesis in normal human cells caused almost no inhibition in fibroblasts of the patient with Down's syndrome. The radioresistant DNA synthesis in cells of this patient was mainly due to a much less inhibition of replicon initiation than that in normal cells. Thus, in the case of Down's syndrome, the cells fail to go through the delays during which DNA lesions can be repaired, unlike the situation being the case in normal cells.