PKU (Phenylketonuria) is a rare and serious inherited metabolic disorder that results when an infant is born without a gene that produces an enzyme important to metabolism. In PKU, the missing enzyme, phenylalanine hydroxylase, is essential for conversion of one amino acid--phenylalanine--to another one--tyrosine. Without the enzyme's catalytic activity, phenylalanine begins to accumulate in the infant's bloodstream, tyrosine begins to decline and, unless the problem is detected during the first few months, mental retardation occurs.

Unfortunately, it was not known until recently that missing or defective enzymes can cause mental and physical disorders. Until the twentieth century, therefore, children with PKU were treated no differently than other severely retarded youngsters. Some remained at home with their parents but many were sent to institutions. Then, in 1934, a Norwegian scientist, Asbjorn Folling, made an important discovery. While studying two mentally retarded siblings, he noticed that their urine turned...