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Fibroblast strains from 12 patients with xeroderma pigmentosum had lower than normal rates of DNA repair, as determined by autoradiographic studies of ultraviolet-induced unscheduled nuclear DNA synthesis. The nuclei in binuclear cells, obtained by fusing fibroblasts from certain pairs of these strains, had a greater rate of DNA On coverslip cultures not treated with virus, grains over "lightly labeled" (1, 2, 5, 6) nuclei from 100 consecutively observed mononuclear cells were counted. Virus-treated coverslips were scamied under low power for areas containing multinucleate cells that had the most light labeling. In such an area, grains were counted over the 100 nuclei of 50 consecu-59 Abbreviations: XP, xeroderma pigmentosum; UV, ultraviolet.

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