Molecular genetics of Huntington's disease.

The discovery of a DNA marker linked to the HD gene has provided new avenues into the investigation of this devastating disorder. Genetic investigations have determined that in most and possibly all HD families, the disease is caused by a defect that maps near the telomere on the short arm of chromosome 4. DNA markers will soon provide presymptomatic diagnosis for this disorder, but this increased capability may be a mixed blessing in the absence of effective treatment. The most hopeful route to developing such treatment lies in cloning and characterization of the primary defect. Precise genetic and physical mapping using DNA markers and improvements in techniques for analyzing large segments of DNA have set the stage for cloning of the disease gene in the near future. It will undoubtedly reveal an interesting mechanism for complete phenotypic dominance in man for comparison with completely dominant mutations in other species, particularly Drosophila. The nature of the defect may provide new insights into the functional organization of the central nervous system. For the sake of the many individuals who are afflicted by HD or who are asymptomatic gene carriers, it is to be hoped that cloning and characterizing the disease gene will also yield the necessary information to develop an effective therapy.