We have cloned and sequenced a full length cDNA for HPRT cDNA for HPRTYale isolated from Lesch-Nyhan subject and identified a single nucleotide substitution which results in amino acid substitution of glycine to arginine. Since most HPRT mutants have normal levels of specific HPRT messenger RNA, mutant cDNA analysis is the method of choice to define the mutation in HPRT deficient subjects.