Deficiency in PAH activity due to mutations in the PAH gene causes hyperphenylalaninemia (HPA), and when blood phenylalanine levels increase above 20 times the normal concentration, the metabolic disease phenylketonuria (PKU) results [REF].
Since phenylketonuria can cause irreversible damage, it is imperative that deficiencies in the Phenylalanine Hydroxylase are determined early on in development.