Cockayne syndrome (CS) 6 is a segmental premature aging syndrome with progressive neurological degeneration (1). CS is caused by mutations in CS complementation groups A (CSA) or B (CSB) genes (2, 3). Approximately 80% of CS patients have mutations in the CSB gene, which encodes a 168-kDa protein belonging to the SWI/SNF2 family of chromatin remodeling proteins (4). Cells from CS patients are hypersensitive to UV radiation-induced DNA damage, and the CSB protein is required for the transcription-coupled nucleotide excision repair of UV radiation-induced DNA lesions (cyclobutane pyrimidine dimers and 6-pyrimidine-4-pyrimidone products) (5). CSB is also believed to play a role in transcription elongation and interacts with the RNA polymerase II elongation complex (6). The molecular basis of the progressive neurological defects in CS patients, however, remains unknown; it has been proposed that neurological symptoms in CS may be due to defective repair and/or processing of oxidative DNA damage in CSB-deficient cells (7).Oxidative DNA damage can be caused by endogenous and exogenous agents. Reactive oxygen species, including highly reactive hydroxyl radicals, are formed as byproducts of normal metabolism, mostly during the process of mitochondrial respiration. It has been estimated that up to 2% of all the O 2 consumed by respiration may be released as reactive oxygen species (8, 9). The central nervous system relies exclusively on mitochondria to generate ATP through oxidative metabolism. As a result, neurons are susceptible to increased levels of oxidative stress, and elevated levels of reactive oxygen species have been implicated in the etiology of neurodegenerative diseases including Alzheimer, Parkinson, and Huntington diseases and amyotrophic lateral sclerosis (for a review, see Ref. 10).Hydroxyl radicals attack DNA bases and the sugar-phosphate DNA backbone, generating modified bases and singlestranded DNA (ssDNA) breaks, respectively (11). Many oxida-