to see if this hypermutability were the result of an 'error-prone ' , excision repair process , we irradiated fibroblasts derived from an xp variant patient , xp4be , under conditions that allowed the cells various lengths of time for excision repair before the onset of dna synthesis ( s phase ) and assayed the frequency of 6-thioguanine ( tg ) -resistant mutants .
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fibroblasts
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the cells