Phenylketonuria (PKU) is a rare inherited disorder that causes an amino acid called phenylalanine to build up in the body. PKU is caused by a change in the phenylalanine hydroxylase (PAH) gene, which helps create the enzyme needed to break down phenylalanine. Without the enzyme necessary to break down phenylalanine, a dangerous buildup can develop when a person with PKU eats foods that contain protein or eats aspartame, an artificial sweetener. Signs and symptoms of untreated PKU can be mild or severe and may include a musty odor in the breath, skin or urine, nervous system problems that may include seizures, skin rashes, such as eczema, lighter skin, hair and eye color than family members, unusually small head size, and hyperactivity. Newborn screening identifies almost all cases of phenylketonuria, and all 50 states in the United States require newborns to be screened for PKU. Treatment for PKU is lifelong and may include a special diet or medication. A phenylalanine screening test identifies levels of phenylalanine in the blood, and newborns receive this test between 24 to 72 hours after birth as part of the newborn screen. If a baby has PKU, they may need testing as often as once a week or more often for the first year of life to check their phenylalanine levels. At first, the baby gets a special protein formula that has reduced phenylalanine, and the amount of phenylalanine in the formula is controlled to meet the babys individual needs. The medicine Kuvan® (sapropterin dihydrochloride) can help some people who have PKU, and the medicine is more likely to work in people with mild or special forms of PKU.