Research in the past decade has uncovered a new class of inherited neurodegenerative diseases, the polyglutamine (polyQ) expansion diseases (1). In each, the underlying mutation is an expansion of a CAG trinucleotide repeat that encodes polyQ in the respective disease proteins (Table 1). All are progressive, ultimately fatal disorders that typically begin in adulthood and progress over 10 to 30 years. The clinical features and pattern of neuronal degeneration differ among the diseases, yet increasing evidence suggests that polyQ diseases share important pathogenic features. In particular, abnormal protein conformation (s) promoted by polyQ expansion seem to be central to pathogenesis (2). PolyQ diseases thus join a growing group of neurodegenerative disorders, including Alzheimer’s disease and many other dementias, in which abnormal protein folding and aggregation are implicated.