Deficiency in 6-pyruvoyl-tetrahydropterin synthase (PTPS) activity is the major cause of tetrahydrobiopterin (BH4)-deficient phenylketonuria. Two single base alterations of PTPS cDNA, a C-to-T transition at nucleotide 259 and a novel A-to-G transition at nucleotide 155 (according to cDNA sequence), were identified in two Chinese PTPS-deficient siblings by the reverse transcription-polymerase chain reaction (RT-PCR). The C-to-T transition at nucleotide 259 results in an amino acid change from proline to serine at codon 87 (Pro87Ser), and the A-to-G transition at nucleotide 155 causes an amino acid change from asparagine to serine at codon 52 (Asn52Ser) of PTPS. The C259T missense mutation can be identified by analysis of the BbvI restriction fragments of the PCR-amplified PTPS cDNA product, and was found to account for 42% (11/26) of 26 Chinese PTPS mutant alleles studied. However, none of 100 normal alleles screened were found to have this change. This result indicates that the C259T transition may be a common mutation in Chinese PTPS-deficient patients.