Hyperphenylalaninemia (HPA), including phenylketonuria (PKU), is a class of diseases caused by the impairment of the enzyme Phenylalanine hydroxylase (PAH), both for mutations in the corresponding PAH gene encoding the enzyme and for deficiency of the BH4 cofactor (Shintaku, 2002). The gene discussed is PAH; the disease is Hyperphenylalaninemia.