Phenylketonuria, also called PKU, is caused by a problem with an enzyme called phenylalanine hydroxylase (PAH). PAH helps break down a chemical called phenylalanine (Phe) in the body. When PAH does not work well, Phe builds up in large amounts and can cause physical and mental problems, including mental retardation.

This registry collects information about patients who have taken, are currently taking, or will take Kuvan to help treat their PKU. The purpose of this registry is to learn more about treating PKU with Kuvan by looking at changes in prescriptions, blood Phe levels, brain function, and other symptoms over time. This registry will also look at the long-term safety and efficacy of Kuvan.