Defective or absent phenylalanine hydroxylase results in buildup of phenylalanine, which is subsequently transaminated to phenylpyruvate and then excreted in the urine. The disease "phenylketonuria" rapidly leads to severe mental retardation if not treated soon after birth with a low phenylalanine diet. Universal screening of newborns in the U.S. for this condition by blood analysis has greatly decreased the morbidity of the untreated condition.