Expansions of CAG trinucleotide repeats encoding glutamine have been found to be the causative mutations of seven human neurodegenerative diseases. Similarities in the clinical, genetic, and molecular features of these disorders suggest they share a common mechanism of pathogenesis. Recent progress in the generation and characterization of transgenic mice expressing the genes containing expanded repeats associated with spinal and bulbar muscular atrophy (SBMA), spinocerebellar ataxia type 1 (SCA1), Machado-Joseph disease (MJD/SCA3), and Huntington's disease (HD) is beginning to provide insight into the underlying mechanisms of these neurodegenerative disorders.