The group of polyQ-associated disorders includes HD, caused by the aggregation of the protein huntingtin (Bates 2003), and Machado–Joseph disease (MJD) (or spinocerebellar ataxia type 3), resulting from the aggregation of the deubiquitylating enzyme ataxin-3 (Schöls et al. 2004). The gene discussed is ATXN3; the disease is Spinocerebellar ataxia type 3.