PAH and phenylketonuria: Phenylketonuria and hyperphenylalaninemia (PKU and HPA; OMIM 261600) are inborn errors of metabolism (IEMs) due to mutations in the PAH gene, normally coding for the liver enzyme phenylalanine hydroxylase (PAH, EC 1.14.16.1), which converts the aminoacid phenylalanine (Phe) into tyrosine (Tyr) [4].