Kuvan Therapy in Phenylketonuria (PKU): The Effect of Blood Phenylalanine Concentration on Kuvan Response

Inclusion Criteria: * all patients with a diagnosis of phenylketonuria * patients who are over the age of 4 years, * patients with a fasting blood phenylalanine ≥600µmol/l Exclusion Criteria: * female patients who are pregnant or attempting to become pregnant * children under four years of age * Concurrent disease of condition that would interfere with study participation or safety such as history of neurological compromise or major medical, psychiatric or learning disorder unrelated to PKU * Any condition that, in the view of the Investigators, places subject at high risk of poor treatment compliance or of not completing the study * Use of L-Dopa, methotrexate, or other drugs that may inhibit folate metabolism