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Cockayne Syndrome (CS) is a human genetic disorder with two complementation groups, CS-A and CS-B. TheCSB gene product is involved in transcription-coupled repair of DNA damage but may participate in other pathways of DNA metabolism. The present study investigated the role of different conserved helicase motifs of CSB in base excision repair. Stably transformed human cell lines with site-directedCSB mutations in different motifs within its putative helicase domain were established. We find that CSB null and helicase motif V and VI mutants had greater sensitivity than wild type cells to γ-radiation. Whole cell extracts from CSB null and motif V/VI mutants had lower activity of 8-hydroxyguanine incision in DNA than wild type cells. Also, 8-hydroxyguanine accumulated more inCSB null and motif VI mutant cells than in wild type cells after exposure to γ-radiation. We conclude that a deficiency in general …