This is the story of the unraveling of a profound medical mystery. It started 70 years ago in Norway, where Mrs Egeland was searching for a physician who could treat her two young children. At birth, both children appeared to be healthy, but they soon developed signs of severe mental retardation. Her search ended when she found Dr. Folling, who determined that the retardation was caused by a new genetic disease, later named phenylketonuria (PKU), that blocks the oxidation of the amino acid phenylalanine. Later, others devised an inexpensive test for PKU and an effective treatment- the low...