Phenylketonuria is caused by a decrease in melanin synthesis due to inhibition of tyrosinase, resulting in a light and brown hair color in children. Phenylketonuria (PKU) is a genetic disorder in which phenylalanine metabolism is impaired due to a deficiency or reduced activity of phenylalanine hydroxylase (PAH) in the liver. It is relatively common among inherited amino acid metabolism deficiency diseases. The diagnosis of this disease should emphasize early diagnosis in order to obtain early treatment to avoid mental retardation. Screening for phenylketonuria must be performed in newborns to obtain an early diagnosis. 1. Screening method: The internationally accepted routine screening method is the bacterial inhibition method discovered by Guthrie. PKU screening kits are available in China. This method is used to estimate the blood phenylalanine level based on the size of the growth band of Bacillus subtilis cultured in the affected child’s blood pair. If the estimated blood phenylalanine level is 0.24 mmol/L, the test is positive. This method can be used in infants 3 to 5 days after birth. Newborns with a family history of phenylalanine should be screened more frequently in the neonatal period. 2.Phenylalanine loading test: This test can directly understand the activity of PAH. The loading dose is 0.1g/kg of oral phenylalanine for 3 days. Blood phenylalanine levels are above 1.22 mmol/L in children with classical PKU and often below 1.22 mmol/L in mild cases. The latter result suggests that these children may be hyperphenylalaninemic without PKU. Etiological diagnosis: The gene that causes phenylketonuria is the PAH gene, and the etiological diagnosis is the detection of PAH gene mutation. the detection of PAH gene mutation can not only make the etiological diagnosis for the patient, but also make the prenatal diagnosis for the fetus. There is a correlation between genotype and phenotype in most patients. The extent to which different mutation types affect PAH activity varies, so testing for mutations in the PAH gene can also be useful in determining prognosis and guiding treatment. There are many methods to detect mutations in the PAH gene, but multiplexed enzyme chain reaction (PCR) is used in combination with one or two of the latter assays, including single-strand conformation polymorphism (SSCP), restriction fragment length polymorphism (RFLP), denaturing gradient gel electrophoresis (DGGE), direct DNA sequencing, mutation site-specific oligonucleotide probes (ASO), PCR-polypropylene amine gel electrophoresis-silver staining, dideoxy fingerprinting, amplificationrefractorymutationsystem (ARMS), enzyme mismatch cleavage methods, etc. Amplified DNA can be analyzed, and SSCP analysis of RNA can be performed. Peripheral blood lymphocytes are used to analyze specimens, and polar bodies (gamete products) can be analyzed for prenatal diagnosis. Analysis of polar bodies and ASO can be used for prenatal diagnosis, and PAH genes with known mutation loci can also be examined by ASO method. There are five most common PAH gene mutations in China: R243Q, Y204C, V399V, Y356X, R413P, and these five PAH gene mutations account for 56.7%. Among the mutations, point mutations were the most common, accounting for 77.4% of the mutation types. Huang Shangzhi proposed a rapid diagnostic procedure for PAH mutations: step 1 for mutation site-specific oligonucleotide probe analysis, the diagnostic rate could reach 66%; step 2 for SSCP analysis of exon 4, the diagnostic rate increased to 80%; step 3 for detection of several common mutation sites, namely R243Q (exon 7), V339V and Y356X (exon 11) by SSCP analysis, which could make a diagnostic rate of 87%. The detection of the PTPS gene was also performed using a PCR-based method combined with the DGGE method, which allowed screening the six coding sequences of this gene and all splice sites of the PTPS gene.