Phenylketonuria (PKU, MIM 261,600) is a deficiency in the hepatic enzyme phenylalanine hydroxylase (PAH; EC 1.14.16.1; OMIM 612,349) that occurs in approximately 1 in 24,000 people, with an estimated 450,000 individuals affected worldwide [1]. The gene discussed is PAH; the disease is phenylketonuria.