Phenylketonuria (PKU; MIM 261600), an autosomal recessive disorder of Phenylalanine (Phe) metabolism, is mainly caused (98 % of cases) by deficient activity of the hepatic enzyme L-phenylalanine-4-hydroxylase (PAH; EC 1.14.16.1), due to mutations in the PAH gene (NM 000277.1). This evidence concerns the gene PAH and phenylketonuria.