Phenylketonuria, often called PKU is a genetic disorder that causes a mutation in the liver (hepatic) enzyme, phenylalanine hydroxylase, that is necessary to metabolize phenylalanine into the amino acid tyrosine. This condition, when untreated, leads to seizures and even mental retardation. However, when diagnosed early can be controlled by a strict diet. Testing for PKU is now standard for newborns.