Phenylketonuria (PKU; OMIM#261600) is probably the best studied and the most frequent inborn error of amino acid metabolism, caused by autosomal recessive deficiency of the hepatic enzyme phenylalanine hydroxylase (PAH; EC 1.14.16.1), catalysing the (6R)-5,6,7,8-tetrahydrobiopterin (BH4) dependent hydroxylation of L-phenylalanine (Phe) to L-tyrosine (Tyr). The gene discussed is PAH; the disease is phenylketonuria.