Nucleotide-excision repair is the repair system for removing bulky lesions from DNA. Humans deficient in this repair pathway suffer from xeroderma pigmentosum (XP), a disease characterized by photodermatoses, including skin cancers. At the cellular level, XP patients fail to remove cyclobutane pyrimidine dimers and pyrimidine(6-4)pyrimidone photoproducts induced by UV light, as well as other bulky DNA lesions caused by various genotoxic agents. XP cells are not particularly sensitive to ionizing radiation or to alkylating agents that cause mostly nonbulky DNA lesions. Therefore, it has generally been assumed that the human nucleotide-excision repair enzyme (excinuclease) is specific for bulky adducts. To determine the substrate range of human excinuclease we used the highly sensitive excision assay and tested bulky adducts, synthetic apurinic/apyrimidinic sites, N6-methyladenine, O6-methylguanine, and mismatches as potential substrates. We found that all of these "lesions" were removed by human excinuclease, although with vastly different efficiencies.