Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant neurodegenerative disorder characterized by ataxia, progressive motor deterioration, and loss of cerebellar Purkinje cells. SCA1 belongs to a growing group of neurodegenerative disorders caused by expansion of CAG repeats, which encode glutamine. Although the proteins containing these repeats are widely expressed, the neurodegeneration in SCA1 and other polyglutamine diseases selectively involves a few neuronal subtypes. The mechanism(s) underlying this neuronal specificity is unknown. Here we show that the cerebellar leucine-rich acidic nuclear protein (LANP) interacts with ataxin-1, the SCA1 gene product. LANP is expressed predominantly in Purkinje cells, the primary site of pathology in SCA1. The interaction between LANP and ataxin-1 is significantly stronger when the number of glutamines is increased. Immunofluorescence studies demonstrate that both LANP and ataxin- 1 colocalize in nuclear matrix-associated subnuclear structures. The features of the interaction between ataxin-1 and LANP, their spatial and temporal patterns of expression, and the colocalization studies indicate that cerebellar LANP is involved in the pathogenesis of SCA1.