Study Overview: This research is a long-term study lasting 10 years for people receiving Palynziq treatment for a condition called phenylketonuria (PKU). PKU is a rare genetic disorder that affects the body's ability to break down an amino acid called phenylalanine, found in many foods. The treatment being studied is called pegvaliase, which helps lower phenylalanine levels in the blood.

Participants will have blood samples taken regularly for testing. Testing will look for immune response, inflammation, and levels of phenylalanine.
**Key Points to Consider:**

• The study lasts for 10 years, with regular blood tests every 3 to 6 months.
• Participants must already be involved in another related study (165-501).
• Participants can leave this study and still stay in the other study (165-501), but not vice versa.

To join, participants must be part of the related study in the US and able to give consent for extra blood tests. Those with legal issues or unable to consent cannot join.