Phenylalanine plays a role in the body's production of melanin, the pigment responsible for skin and hair color. Therefore, infants with the condition often have lighter skin, hair, and eyes than brothers or sisters without the disease.

Other symptoms may include:

• Delayed mental and social skills
• Head size significantly below normal
• Hyperactivity
• Jerking movements of the arms or legs
• Mental retardation
• Seizures
• Skin rashes
• Tremors
• Unusual positioning of hands

If the condition is untreated or foods containing phenylalanine are not avoided, a "mousy" or "musty" odor may be detected on the breath and skin and in urine. The unusual odor is due to a build up of phenylalanine substances in the body.

PKU can be easily detected with a simple blood test. Most states require a PKU screening test for all newborns. The test is generally done with a heelstick shortly after birth.

If the initial screening test is positive, further blood and urine tests are required to confirm the diagnosis.