This is a 10-year multi-center, prospective, longitudinal, single arm study evaluating immunologic, inflammatory and laboratory parameters associated with long-term Palynziq treatment in subjects with phenylketonuria (PKU) in the United States (US). Subjects in the US for whom a clinical decision has been made that they will receive pegvaliase to treat their PKU within 30 days following the date of enrollment in Study 165-501 (incident-users) or who have previously started treatment with pegvaliase at the date of enrollment in Study 165-501 (prevalent-users) are eligible for participation in Study 165-503.

This is a 10-year multi-center, prospective, longitudinal, single arm study evaluating immunologic, inflammatory and laboratory parameters associated with long-term Palynziq treatment in subjects with phenylketonuria (PKU) in the United States (US). Subjects in the US for whom a clinical decision has been made that they will receive pegvaliase to treat their PKU within 30 days following the date of enrollment in Study 165-501 (incident-users) or who have previously started treatment with pegvaliase at the date of enrollment in Study 165-501 (prevalent-users) are eligible for participation in Study 165-503. Subjects participating in the 165-501 study who consent to participate in this study (165-503) will be requested to provide pre-dose blood samples for Phe, immunologic and inflammatory marker testing approximately every 3 months for the first 3 years of participation, then every 6 months for the remainder of the study. Subjects will be requested to provide additional pre-dose blood samples at the time of Study 165-501 -protocol defined safety events. Additional pre-dose samples will be drawn as per standard of care as detailed in protocol 165-501. The blood Phe, immunologic (PAL IgG, PEG IgG, PEG IgM, anti-pegvaliase IgE), and inflammatory (C3/C4, hsCRP) markers, will be sent to a central laboratory for processing. Data collected in the 165-501 study will be combined with the data collected in this study to decrease burden on sites for data entry and to avoid duplication of ADR reporting (see Criteria for Evaluation). The combined data will be reported in the Clinical Study Report for this study. Subjects can withdraw from Study 165-503 and remain on Study 165-501, but they cannot withdraw from Study 165-501 and remain on Study 165-503 as safety events and other data-points are collected in Study 165-501.

Inclusion Criteria:

• Subjects enrolled at US sites participating in the 165-501 study.

Exclusion Criteria:

• Legal incapacity or limited legal capacity without legal guardian representation.
• Subject is unable or unwilling to provide informed consent for the additional interventional burden of the study (blood sampling).

Subjects enrolled at US sites participating in the 165-501 study.