This study is for people with phenylketonuria (PKU), a rare condition where the body can't break down a substance called phenylalanine (Phe) found in foods. The study is about a medicine called **Palynziq**, also known as **pegvaliase**, which helps people with PKU by lowering Phe levels. It will take place over 10 years at different centers in the United States. Participants will have to give blood samples regularly: every 3 months for the first 3 years and then every 6 months after. These samples will help doctors understand how the medicine affects the immune system and inflammation in the body. Only people already in another study (165-501) can join this one. They can leave this study anytime but must stay in the initial study to keep getting their treatment safely.

• Study lasts for 10 years with regular blood tests.
• Open to those already in the related PKU study (165-501).
• Participants can leave this study but not the related study.