The expansion of trinucleotide CAG repeat sequences has been shown to be the underlying cause of eight human neurodegenerative disorders, including Huntington’ s disease (HD), spinal and bulbar muscular atrophy (SBMA), dentatorubral-pallidoluysian atrophy (DRPLA), Machado-Joseph disease (MJD), and the spinocerebellar ataxias (SCAs)(for review see refs. [1 –3 ]). These diseases are progressive disorders characterized by motor and/or cognitive impairments and distinctive pathological patterns of neuronal degeneration within the central nervous system (CNS). Each is caused by CAG codon expansion within a unique gene producing a polyglutamine (polyGln) tract enlargement in the target protein (4 –6 ). All of these neurodegenerative disorders present a common feature: the accumulation of the polyGln repeat disease-related protein into neuronal intranuclear inclusions, which have become the neuropathological signature of polyGln disorders (7 –9 ). Several lines of reasoning suggest strongly that the expanded polyGln tract is itself responsible for the pathogenesis. First, the disease-related proteins do not present any homology in either size or amino acid composition outside of the expanded polyGln tract (6 ,10 –12 ). Second, the severity of the disease increases with the length of the polyGln repeat. Third, the length of the polyGln sequence that triggers increased risk for developing the pathology is very similar for almost of these diseases (2 ,13 ,14 ). Finally, transgenic animals expressing protein fragments with an expanded polyGln repeat exhibit neurodegenerative phenotypes reminiscent of disease symptoms (15 –19 ).