Genetics of Huntington disease.

In this chapter, we review the evolution of our understanding of the genetic aspects of HD, and the applications of our understanding in the management of Huntington's disease patients and families over the last 150 years. Important aspects of the clinical genetics and epidemiology of Huntington's disease are discussed, such as the definition of "normal" and "abnormal" numbers of CAG (cytosine-adenine-guanine) repeats in the critical spot within the huntingtin gene, meiotic instability of CAG repeat numbers, common Huntington's disease genetic haplotypes, compound heterozygosity for an abnormal gene, and somatic mosaicism for CAG repeat expansions. We touch only briefly on the creation of multiple animal models for Huntington's disease that have profoundly impacted our understanding of the disease and permitted the development of potential disease-modifying treatments, and end with what is, at the time of writing, the dawn of a new era: the advent of gene-based therapies (gene silencing, gene editing) for Huntington's disease.