Phenylketonuria (PKU) is a rare, inherited, metabolic disorder that can result in mental retardation and other neurological problems. People with this disease have difficulty breaking down and using (metabolizing) the amino acid phenylalanine. PKU is sometimes called Folling's disease in honor of Dr. Asbjorn Folling who first described it in 1934.

Phenylalanine is an essential amino acid. These substances are called "essential" because the body must get them from food to build the proteins that make up its tissues and keep them working. Therefore, phenylalanine is required for normal development. Phenylalanine is a common amino acid and is found in all natural foods. However, natural foods contain more phenylalanine than required for normal development. This level is too high for patients with PKU, making a special low-phenylalanine diet a requirement.

The incidence of PKU is approximately one in every 15,000 births (1/15,000). There are areas in the world where the incidence...