PKU PARTICIPANTS (ARM 1):

• Subject has a confirmed diagnosis of PKU with hyperphenylalaninemia documented by a fasting Phenylalanine level of at least 360 umol/L (6 mg/dL)
• Patient is at least 4 years old (there is no upper age limit for this study)
• Willing and able to provide written authorization or, if under the age of 18 years, provide written assent (if required) and written patient authorization by a parent or legal guardian
• Willing to undergo study related procedures including commencing Kuvan treatment for patient not currently on treatment; temporary discontinuation of Kuvan for patient on treatment; and completing the 24-Hour Blood Assessment
• Authorized to provide personal health information
• Subjects should not be pregnant and willing to use appropriate birth control during the study

CONTROL GROUP (ARM 2):

• Healthy, non-PKU individuals. They will be age-sex matched to the PKU group. They may be a relative (ex: sibling) of a PKU participant, but they don’t have to be a blood relation.
• Individual is at least 4 years old (there is no upper age limit)
• Willing and able to provide written consent or, if under the age of 18 years, provide written assent and written participant authorization by a parent or legal guardian
• Authorized to provide personal health information

Exclusion Criteria (BOTH ARMS):

• Subjects who do not meet all the inclusion criteria
• Age < 4 years
• Concomitant medical problems or medications which at the discretion of the principal investigator would put participant at health risk or prevent them from completing study.
• If female, unwillingness to use birth control during the period of the study drug administration (this doesn’t apply to Arm 2