PAH and phenylketonuria: Phenylketonuria (PKU; OMIM #261600) is an autosomal recessive genetic disorder caused by the deficiency in the hepatic enzyme phenylalanine hydroxylase (PAH; OMIM #612349) (Scriver et al., 1995) that converts phenylalanine into tyrosine requiring the cofactor tetrahydrobiopterin (BH4).