Spinocerebellar ataxia type 3 (SCA3), also known as Machado–Joseph disease (MJD), is a progressive neurodegenerative disease caused by cytosine–adenine–guanine (CAG) repeat expansion of the ATXN3 gene encoding ataxia protein-3. The gene discussed is ATXN3; the disease is Spinocerebellar ataxia type 3.