Early in 1993, an unstable, expanded trinucleotide repeat in a novel gene of 
unknown function was identified on HD chromosomes. This discovery unleased a 
flurry of experimentation that has established the expanded CAG repeat the 
almost universal cause of the characteristic neurologic symptoms and pathology 
of this neurodegenerative disorder of midlife onset. The biochemical basis for 
the specific neuronal loss of HD remains uncertain, but the genetic lesion 
probably acts via its consequent polyglutamine segment in the protein product, 
huntingtin. This review will describe the basic parameters of the HD repeat's 
behavior and the knowledge that has accumulated concerning its potential 
mechanisms of action.