Nine dominantly inherited neurodegenerative diseases are caused by expansion of a CAG repeat encoding glutamine. An important development in the study of such "polyglutamine" diseases was the realization that merely shutting off expression of a disease-encoding transgene could arrest progression in animal models with significant disease pathology. Such studies opened the door to a powerful new therapeutic approach now being pioneered: silencing of the dominant disease allele by RNA-mediated interference (RNAi), for the arrest—and potential reversal—of the disease process.