HPRT1 has a central role in the generation of purine nucleotides through the purine salvage pathway. HPRT1 catalyzes conversion of hypoxanthine to inosine monophosphate and guanine to guanosine monophosphate via transfer of the 5-phosphoribosyl group from 5-phosphoribosyl 1-pyrophosphate. The Lesch-Nyhan syndrome is particularly favorable for this purpose because no affected males reproduce, the diagnosis is unequivocal and cases come readily to attention, and particularly because heterozygosity can be demonstrated in females by the existence of 2 populations of cultured fibroblasts. There were few new mutations, contrary to the expected one-third. On the other hand, about one-half of heterozygous females were new mutations, as is predicted by theory.