ATXN3 is a polyglutamine (polyQ)-containing protein that carries 14–41 polyQ repeats in the normal population; when the polyQ length expands beyond 60, it causes Spinocerebellar ataxia type 3 (SCA3; OMIM:109150), also known as Machado-Joseph Disease (MJD), a hereditary neurodegenerative disorder characterized by gait ataxia, dysarthria and ophthalmoplegia, variably associated with spasticity, dystonia or amyotrophy and peripheral neuropathy [23]. The gene discussed is ATXN3; the disease is Machado-Joseph disease.