Disorders primarily affecting the nervous system comprise approximately one third of all established Mendelian genetic diseases in man. Recombinant DNA technology provides new approaches to the diagnosis and elucidation of the molecular pathology of these disorders. For a small but increasing number of disorders the DNA sequence coding for the involved protein has been used to define the precise molecular defect. An example is the Lesch-Nyhan syndrome. In many other situations, DNA fragments located near to the mutant gene can be used in family linkage studies to determine who is likely to have inherited the abnormal allele(s). Examples include Duchenne muscular dystrophy, Huntington's disease, and phenylketonuria. This technology offers unique opportunities to investigate the function of the nervous system in health and disease and will have a major impact on the neurosciences and the practice of clinical neurology.