Spinocerebellar ataxia type 3 (SCA3), also known as Machado-Joseph disease, is caused by an unstable CAG repeat expansion in the ataxin-3 (ATXN3) gene leading to an expansion of polyglutamines in the C-terminus of the corresponding protein, ataxin-3 [72,76]. The gene discussed is ATXN3; the disease is Spinocerebellar ataxia type 3.