Phenylketonuria (PKU, OMIM 261600) is an autosomal recessive inborn error of phenylalanine metabolism, caused by a deficient activity of phenylalanine hydroxylase (PAH, EC 1.14.16.1) – an enzyme that coverts the essential amino acid phenylalanine (Phe) to tyrosine, leading to an accumulation of Phe in blood and other tissues (1). Here, PAH is linked to phenylketonuria.