Early-onset ataxia with ocular motor apraxia and hypoalbuminemia (EAOH) is an autosomal recessive neurodegenerative disorder characterized by early-onset ataxia, ocular motor apraxia, and hypoalbuminemia. Recently, the causative gene for EAOH, APTX, has been identified. Of the two splicing variants of APTX mRNA, the short and the long forms, long-form APTX mRNA was found to be the major isoform. Aprataxin is mainly located in the nucleus, and, furthermore, the first nuclear localization signal located near the amino terminus of the long-form aprataxin is essential for its nuclear localization. We found, based on the yeast two-hybrid and coimmunoprecipitation experiments, that the long-form but not the short-form aprataxin interacts with XRCC1 (x-ray repair cross-complementing group 1). Interestingly the amino terminus of the long-form aprataxin is homologous with polynucleotidekinase-3'-phosphatase, which has been demonstrated to be involved in base excision repair, a subtype of single-strand DNA break repair, through interaction with XRCC1, DNA polymerase beta, and DNA ligase III. These results strongly support the possibility that aprataxin and XRCC1 constitute a multiprotein complex and are involved in single-strand DNA break repair, and furthermore, that accumulation of unrepaired damaged DNA underlies the pathophysiological mechanisms of EAOH.