Phenylketonuria (PKU) is an autosomal recessive genetic disorder characterized by a deficiency in the enzyme phenylalanine hydroxylase (PAH). This enzyme is necessary to metabolise the amino acid phenylalanine to the amino acid tyrosine. When PAH is deficient, phenylalanine accumulates on the brain tissue and is converted into phenyl pyruvate (also known as phenyl ketone), which is detected in the urine. PAH is found on chromosome number 12. If left untreated, this condition can cause problems with brain development, leading to progressive mental retardation and seizures. However, PKU can be controlled by a special diet with low in phenylalanine and high in tyrosine can be a very effective treatment. There is no cure. The disorder is only detectable after irreversible damage is done hence early diagnosis is crucial. Most babies in developed countries are screened for PKU soon after birth. PKU is usually detected using the HPLC test, but some clinics still use the Guthrie test, as a part of national biochemical screening programs.