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The first diagnosed cases of Phenylketonuria (PKU) were identified in 1934 by Norwegian doctor Asbjorn Folling. Dr. Folling found that the urine of two of his young mentally handicapped patients contained a high level of phenylalanine. Follwing this discovery, it was found that the absence or malfunction of the phenylalanine hydroxylase enzyme is due to the mutation of the ''PAH'' gene  and inherited autosomal recessively. This may result in a genetic disorder known as Phenylketonuria (PKU). This information was not utilized until the early 1950s when it was found that under a low phenylalanine diet, some of the symptoms found in children suffering from PKU could be reversed. Due to a diet rich in phenylalanine, this enzyme is vital in the regulation in phenylalanine plasma concentration by converting about 75% of the amino acid to tyrosine. Excessive amounts of phenylalanine has been shown to cause mental retardation in humans. Presently, it is regulation to screen newborns children for phenylketonuria with a simple blood or urine test. Treatment for such a disease is a low phenylalanine diet, avoidance of the sweetener Aspartame, and early detection. PKU can also be caused by a deficiency in or inability to regenerate tetrahydrobipternin, the cofactor essential to the function of PheOH. Although this is not usually the cause of PKU, patients can be treated by taking tetrahydrobiopterin supplements. <ref> January 2005: Phenylalanine Hydroxylase [http://www.pdb.org/pdb/education_discussion/molecule_of_the_month/download/PhenylalanineHydroxylase.pdf]</ref>