Problems in the digestion of phenylalanine (phe) to tyrosine (tyr) lead to the buildup of both phe and phenylpyruvate, in a disease called Phenylketonuria (PKU).  These two compounds build up in the blood stream and cerebral spinal fluid, which can lead to mental retardation if left untreated.  Treatment consists of a restricted diet of foods that contain phe or compounds that can breakdown into phe.  Children in the US are routinely tested for this at birth.  For more information see the Phenylketonuria page or the link below.