The Rate of Removal of Sunlight‐induced Pyrimidine Dimers From the Dna of Cultured Human Diploid Fibroblasts

The rate of excision of sunlight‐induced pyrimidine dimers in DNA of exposed human cells was determined. Two normal excision repair‐proficient human diploid fibroblast strains (WS‐1 and KD) and a repair‐deficient strain (XP12BE, group A) maintained in a nondividing state were exposed to summer noon‐time sunlight for times (5 and 20 min) that induced numbers of dimers equivalent to far UV (254 nm) exposures of 1 and 4 J/m 2 . Pyrimidine dimers were quantified in extracted DNA using a U V‐endonuclease‐alkaline sedimentation assay. The excision rates of these dimers were similar to those observed for the excision of UV‐induced pyrimidine dimers. No sunlight‐induced inhibition or stimulation of DNA repair was observed in either strain at these low exposures.