CAG trinucleotide extensions of 41-81 repeats within the coding region of ATXN1 are responsible for the autosomal dominant spinocerebellar ataxia 1 (SCA1 [MIM 164400]), a neurodegenerative disorder with progressive cerebellar degeneration. The gene discussed is ATXN1; the disease is spinocerebellar ataxia type 1.