Phenylketonuria (PKU) is an autosomal recessive inborn error of metabolism in which a person cannot metabolize phenylalanine (Phe) into tyrosine and its constituent components. As a result, Phe accumulates in the blood, causing widespread brain damage, consequent severe mental retardation, and other organic problems. Before birth, nutrients are metabolized by the mother and transmitted through the umbilical cord, so damage begins only when the new¬ born begins to feed. In the 1960s, national screening of newborns for PKU and treatment of affected individuals with a synthetic diet low in Phe prevented much damage associated with this disorder. The initial protocol called for affected individuals to stay on the diet until late childhood or early adolescence, after which they could eat normal food because brain development was thought to be complete. However, the diet did not cure the disorder, and Phe then began to accumulate in their blood. Most treated adults have low average intelligence and live near-normal lives.