QDPR and dihydropteridine reductase deficiency: Globally, PTPS deficiency is the most frequent BH4 deficiency (≈54%), followed by dihydropteridine reductase (DHPR) deficiency (≈33%).[23] However, in China, PTPS deficiency accounts for ≈96% of BH4 deficiencies, followed by DHPR deficiency (2.4%) and guanosine triphosphate cyclohydrolase I (GTPCH) deficiency (1.6%).[24] Notably, the prevalence of BH4 deficiency is 3.8 per 1 000 000 live births in China.[25] These reasons resulted in almost no BH4 samples with PTPS deficiency, GTPCH deficiency, and other types of BH4 deficiencies except for PTPS deficiency being collected in the hospital.