Phenylketonuria (PKU, OMIM 261600) is an inborn error of phenylalanine (Phe) metabolism caused in most cases (98%) by an inherited deficiency in l-phenylalanine-4-hydroxylase (PAH; EC 1.14.16.1) activity, leading to elevated levels of Phe in body fluids [1]. Here, PAH is linked to phenylketonuria.