MJD is caused by the expansion of an unstable CAG tract in exon 10 of the ATXN3 gene encoding for ataxin-3 (ATXN3) and is responsible for C-terminal expanded polyglutamine (polyQ) stretch in mutant ATXN3 (mATXN3). This evidence concerns the gene ATXN3 and Spinocerebellar ataxia type 3.