Phenylketonuria (PKU) is one of the commonest inherited disorders - occurring in approximately 1 in 10,000 babies born in the U. S. It occurs in babies who inherit two mutant genes for the enzyme phenylalanine hydroxylase (PAH). This enzyme normally breaks down molecules of the amino acid phenylalanine that are in excess of the body's needs for protein synthesis.

All babies born in the U.S. are tested for PKU at birth. If not diagnosed and managed properly, PKU can result in mental retardation, epilepsy and other neurologic symptoms.

The primary treatment is avoidance of phenylalanine and a low protein diet, which is why foods are required to be labeled for the benefit of phenylketonurics who must always be vigilant about their diet.