Inclusion Criteria:

• 8 years of age and older
• Prior successful participation in Study PKU-003
• Willing and able to provide written informed consent or assent and written informed consent (if required) by a parent or legal guardian
• For females of child-bearing potential only: Negative urine pregnancy test within 24 hours prior to enrollment. Women using acceptable birth control measures must agree to continue to use those measures while participating in the study
• Willing and able to comply with study procedures
• Willing to continue current diet unchanged while participating in the study

Exclusion Criteria:

• Perceived to be unreliable or unavailable for study participation or, if under the age of 18, have parents or legal guardians who are perceived to be unreliable or unavailable
• Withdrew from, or otherwise did not successfully complete, study PKU-003, except for subjects who were removed from the study because their blood Phe exceeded the alert level
• Expected to require any investigational agent or vaccine prior to completion of all scheduled study assessments
• Pregnant or breastfeeding, or planning pregnancy
• Concurrent disease or condition that would interfere with study participation or safety (e.g., seizure disorder, oral steroid-dependent asthma or other condition requiring oral or parenteral corticosteroid administration, or insulin-dependent diabetes)
• Requirement for concomitant treatment with any drug known to inhibit folate synthesis (e.g., methotrexate)
• Concurrent use of levodopa