Spinocerebellar ataxia type 1 (SCA1) is a neurodegenerative disorder caused by expansion of a CAG trinucleotide repeat. In this study, we describe the identification and characterization of the gene harbouring this repeat. The SCA1 transcript is 10, 660 bases and is transcribed from both the wild type and SCA1 alleles. The CAG repeat, coding for a polyglutamine tract, lies within the coding region. The gene spans 450 kb of genomic DNA and is organized in nine exons. The first seven fall in the 5′ untranslated region and the last two contain the coding region, and a 7, 277 basepairs 3′ untranslated region. The first four non–coding exons undergo alternative splicing in several tissues. These features suggest that the transcriptional and translational regulation of ataxin–1, the SCA1 encoded protein, may be complex.