1 Introduction Spinocerebellar Ataxia type 3 (SCA3), also known as Machado–Joseph disease (MJD), is a rare, neurodegenerative, hereditary ataxia with no disease-modifying treatments. SCA3 results from the expansion of a trinucleotide CAG repeat in the ATXN3 gene, which is translated into an expanded polyglutamine (polyQ) tract in the ataxin-3 protein (Atx3).