Identification by the many scientists in The Huntington's Disease Collaborative Research Group, including Gusella, of the single defective gene on chromosome 4 that causes the progressive brain disorder, Huntington's disease. The defect is dominant, meaning that anyone who inherits it from a parent with Huntington's will eventually develop the disease. The defective gene codes for a protein called huntingtin. Since identification of the defective gene, a diagnostic genetic test has been developed that can detect the defective gene in people who do not yet have symptoms of the disease.