Genetic and Molecular Studies

Huntington’s disease (HD) is a rare inherited neurologic disorder due to a single mutational mechanism in a large gene (HTT). The mutation is an abnormal CAG repeat expansion, which is translated to a polyglutamine stretch in the huntingtin protein. The growing field of repeat expansion disorders benefits greatly from the lessons learned from the role of the CAG repeat expansion in HD and its resulting phenotype–genotype correlations. The molecular diagnosis can be difficult, and there are some pitfalls for accurate sizing of the CAG repeat, especially in juvenile HD and for intermediate alleles. Correlation between CAG length and age of onset accounts for up to 72% of the variance in different populations, but the search for genes modifying age of onset or progression of HD is still ongoing.