ATXN3 and Spinocerebellar ataxia type 3: Importantly, the expansion of the CAG trinucleotides within the ATXN3 gene leads to an expanded polyglutamine domain in the encoded protein, which has been associated with the onset of the spinocerebellar ataxia type 3, also known as Machado–Joseph disease, the most common dominantly inherited ataxia worldwide.