We have investigated the molecular basis of hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency in a patient who presented with the Lesch-Nyhan syndrome. A catalytically incompetent form of HPRT has been isolated from this patient's erythrocytes and lymphoblasts. This enzyme variant, which we have termed HPRTKinston, is indistinguishable from the normal enzyme in terms of its intracellular concentration and maximal velocity, but differs with respect to its isoelectric point (more basic) and Michaelis constants for both substrates (markedly elevated). The tryptic peptides of HPRTKinston were mapped by reverse-phase high pressure liquid chromatography in an attempt to define the precise abnormality in its primary structure. Sequence analysis of the single aberrant tryptic peptide in HPRTKinston revealed an aspartic acid to asparagine amino acid substitution at position 193. Electrophoretic analysis of the CNBr peptides of HPRTKinston confirmed the location of the proposed mutation. This amino acid substitution can be explained by a single nucleotide change in the codon for aspartic acid 193 (GAC leads to AAC). This is the first specific mutation described at the molecular level in a patient with the Lesch-Nyhan syndrome.