Hypoxanthine-guanine phosphoribosyltransferase (HPRT; IMP: pyrophosphate phosphoribosyltransferase, EC 2.4.2.8) functions in the purine-metabolic salvage pathway. Two clinical syndromes are associated with a deficiency in HPRT enzyme activity. Virtually complete deficiency leads to the Lesch-Nyhan syndrome, whereas partial deficiency results in hyperuricemia and severe gouty arthritis. Marked heterogeneity in the mutations leading to HPRT deficiency has been found. Mutant enzymes vary with respect to levels of HPRT immunoreactive protein, electrophoretic migration, kinetic properties and amino acid sequence. Analysis of DNA and RNA from patients with HPRT deficiency has revealed point mutations, an internal gene duplication and partial as well as complete gene deletions accounting for the various HPRT mutant enzymes.