PKU is a treatable autosomal recessive inherited metabolic disease and is categorized as per the different etiologies into PKU caused by mutations in the PAH gene resulting in decreased or loss of PAH activity and BH4D deficiency caused by a congenital defect of an enzyme in the synthesis or metabolism pathway of the coenzyme of PAH (BH4) [8, 9]. The gene discussed is PAH; the disease is metabolic disease.