Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) is a salvage pathway enzyme for purine metabolism (another is specific for adenine). When a defect in HGPRT reduces its activity to a low level, gout is the result. When the defect leads to the complete absence of activity of HGPRT, Lesch-Nyhan syndrome is the result. The gene for HGPRT is found on the X chromosome, so the disease is sex-linked. Patients have severe "gouty" arthritis and a dramatic malfunction of the nervous system, manifested as behavioral disorders, learning disabilities, and hostile or aggressive behavior, often self-directed. Individuals with Lesch-Nyhan syndrome rarely live beyond 20 years.