The normal value of phenylalanine in newborns is 0.06-0.18 mmol/L or 1-3 mg/dL. If the phenylalanine concentration in newborns is within this range, it is normal. If the phenylalanine level is significantly high, it is considered to be due to phenylketonuria. Newborns with phenylketonuria will have neurological abnormalities, such as excitability, hyperactivity and aggressive behavior. In addition, the appearance of the newborn is very specific, as the newborn has lighter hair and skin color, and 1/3 of the newborns have dry skin and eczema, and the newborn’s urine has a distinctive rat urine odor, which is also known as rat urine. A low-phenylalanine diet is required for newborns with high phenylalanine.