PAH and phenylketonuria: Phenylketonuria (PKU; OMIM 261600) is the most prevalent inborn error of the metabolism (IEM) caused by mutations in the phenylalanine hydroxylase (PAH, OMIM * 612349) gene [1] which catalyzes the hydroxylation of phenylalanine (Phe) to tyrosine (Tyr) using tetrahydrobiopterin (BH4) as a cofactor [2].