DNA trinucleotide repeat expansion diseases represent an interesting group of disorders that include a common cause of mental retardation and autism as well as neurodegenerative and other diseases. Many of these disorders have expression in the pediatric age group. The varied molecular mechanisms of these disorders make them model diseases for the study of mitochondrial dysfunction induced apoptosis, abnormal axonal transport induced apoptosis and disrupted transcription of neighboring genes. Clinical variation in the pathogenesis, severity, onset and inheritance of these disorders make them models for clinical study and research.