In 1960, Robert Guthrie, a microbiologist and physician working in Buffalo, New York, devised a simple way to measure the level of phenylalanine in the blood of children with the genetic disorder phenylketonuria (PKU). Guthrie's interest in the problem was intensely personal — he had a son and a niece who were mentally retarded. The niece's condition was due to PKU, in which a defective liver enzyme results in an inability to metabolize phenylalanine, an essential amino acid found in all dietary proteins.
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