Phenylketonuria (PKU), fehn uhl kee tuh NYUR ee uh, is a hereditary disease that results in mental retardation unless treated during early infancy. The bodies of people with PKU accumulate too much phenylalanine, one of the amino acids that make up proteins. PKU is caused by a mutation (change) in a gene that alters the function of the enzyme phenylalanine hydroxylase. This enzyme normally converts phenylalanine to the amino acid tyrosine. In PKU, phenylalanine consumed in food is not properly converted to tyrosine, and phenylalanine accumulates. Excess phenylalanine interferes with the growth, development, and function of the brain.