Expansion of the polyQ tract above a threshold of 45–55 residues leads to ataxin-3 self-assembly and triggers spinocerebellar ataxia type 3 (SCA3), also known as Machado–Joseph disease, a highly incapacitating autosomal-dominant neurodegenerative disorder. The gene discussed is ATXN3; the disease is Spinocerebellar ataxia type 3.