Genome analyses suggested that archaea could possess a eukaryotictype nucleotide excision repair (NER) pathway because they encode homologs of several eukaryotic NER proteins.To investigate the archaeal nucleotide excision repair pathway in vivo,the genes encoding the potential NER helicases in the genome of the hyperthermophilic crenarchaeon Sulfolobus islandicus were deleted via a gene knockout strategy and deletion mutants of the xpb1 and xpb2 genes were isolated,indicating that they were not essential to the survival of S. islandicus.Phenotypic analysis of deletion mutants showed that the xpb1 mutant was only slightly sensitive to 4NQO,as compared with the wildtype strain REY15A,while the xpb2 mutant showed no sensitivity to 4NQO.It is the first genetic study of the crenarchaeon NER pathway.