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Ataxia cause identified An investigation into the molecular basis of the disease SCAN1 (spinocerebellar ataxia with axonal neuropathy-1) has identified for the first time a defect in the repair of chromosomal single-strand breaks in a neurodegenerative disease. The disease results from mutations in tyrosyl phosphodiesterase 1, but the known function of this enzyme — repairing double-strand breaks during replication — seemed unlikely to cause the observed pathology. The new study reveals a second function for the enzyme in human cells: repairing chromosome breaks caused by oxidative stress in post-mitotic neurons, and it is this that is likely to cause the symptoms of SCAN-1. Spinocerebellar ataxia with axonal neuropathy-1 (SCAN1) is a neurodegenerative disease that results from mutation of tyrosyl phosphodiesterase 1 (TDP1) 1 . In lower eukaryotes, Tdp1 removes topoisomerase 1 (top1) peptide from DNA termini during the repair of double-strand breaks created by collision of replication forks with top1 cleavage complexes in proliferating cells 2 , 3 , 4 . Although TDP1 most probably fulfils a similar function in human cells, this role is unlikely to account for the clinical phenotype of SCAN1, which is associated with progressive degeneration of post-mitotic neurons. In addition, this role is redundant in lower eukaryotes, and Tdp1 mutations alone confer little phenotype 4 , 5 , 6 , 7 . Moreover, defects in processing or preventing double-strand breaks during DNA replication are most probably associated with increased genetic instability and cancer, phenotypes not observed in SCAN1 (ref. 8 ). Here we show that in human cells TDP1 is required for repair of chromosomal single-strand breaks arising independently of DNA replication from abortive top1 activity or oxidative stress. We report that TDP1 is sequestered into multi-protein single-strand break repair (SSBR) complexes by direct interaction with DNA ligase IIIα and that these complexes are catalytically inactive in SCAN1 cells. These data identify a defect in SSBR in a neurodegenerative disease, and implicate this process in the maintenance of genetic integrity in post-mitotic neurons.