Huntington's disease is an autosomal dominant neurodegenerative disorder caused by an expanded CAG repeat. It is characterized by motor and cognitive disturbances, as well as cellular dysfunction and loss in the basal ganglia and the cerebral cortex. The mutant protein huntingtin aggregates in cells. The toxicity of mutant huntingtin, or the loss of its normal function, causes disruption of cellular functions such as protein and calcium metabolism, transmitter release, mitochondria and gene transcription. Recent findings in basic research open up new possibilities for novel therapies.