Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant neurodegenerative disorder caused by expansion of a GAG trinucleotide repeat which codes for glutamine in the protein ataxin–1. We have investigated the effect of this expansion on ataxin–1 by immunoblot analysis. The wild–type protein is detected in both normal and affected individuals; however, a mutant protein which varies in its migration properties according to the size of the GAG repeat is detected in cultured cells and tissues from SCA1 individuals. The protein has a nuclear localization in all normal and SCA1 brain regions examined but a cytoplasmic localization of ataxin–1 was also observed in cerebellar Purkinje cells. Our data show that in SCA1, the expanded alleles are faithfully translated into proteins of apparently normal stability and distribution.