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Medical Content Created by the Faculty of the
Harvard Medical School

Phenylketonuria (PKU)

What Is It?

Phenylketonuria (PKU) is a rare genetic (inherited) disorder that can cause abnormal mental and physical development if not detected promptly and treated appropriately. Normally, when a person eats foods that contain protein, special chemicals called enzymes break down these proteins into amino acids and even smaller parts, which are the body's building blocks for growth and repair. A person with PKU does not have enough of the specific enzyme that breaks down the amino acid phenylalanine. Therefore, any phenylalanine in the food someone eats cannot be digested properly and collects in the body.

Too much phenylalanine in the body causes problems with the brain and other organs. Damage from a buildup of phenylalanine can begin within the first month of life and, if undetected and/or untreated, PKU results in severe mental retardation, hyperactivity, and seizures.

PKU affects about 1 baby in every 10,000 or 15,000 births. To be born with PKU, a baby has to have inherited the PKU gene from both parents. Most often, the parents do not know that they carry the gene.