IntroductionNeurodegenerative diseases have been defined as diseases characterized by gradual loss of neurops in particular regions of the central nervous system due to "unknown" causes. Substantial number of neurodegenerative diseases exhibit familial occurrence, suggesting that mutations in the causative genes are the primary cause of the diseases. Application of the molecular genetic strategy of "positional cloning" has enabled us to identify the causative genes without prior knowledge of the pathophysiology of the diseases. Among the causative genes which have been identified by the strategy of positional cloning, unstable expansions of triplet repeats have recently been discovered to be a common novel disease mechanism for neurodegenerative diseases. In this review I will focus on recent developments in the research on triplet repeat diseases with particular emphasis on non-Mendelian aspects of triplet repeat diseases.