PAH and phenylketonuria: Phenylketonuria (PKU; OMIM #261600) is an autosomal recessive inborn error of phenylalanine metabolism caused by wide range of mutations of the phenylalanine hydroxylase (PAH) gene located on chromosome 12q22–24.1 coding for the hepatic enzyme PAH that converts phenylalanine into tyrosine in the presence of molecular oxygen and catalytic amounts of tetrahydrobiopterin (BH4).1