Phenylketonuria (PKU; Online Mendelian Inheritance in Man, OMIM 261,600) is an autosomal recessive disorder of phenylalanine (Phe) metabolism [1], primarily due to mutations in Phe hydroxylase (PAH) gene, which facilitates conversion of the essential amino acid Phe to tyrosine. The gene discussed is PAH; the disease is phenylketonuria.