Response to Kuvan® in Subjects With Phenylketonuria (PKU) in a 4 Weeks Testing Period

Inclusion Criteria: * Subjects aged 4 years or older at the time the informed consent is obtained * Subjects diagnosed with PKU (subgroups defined as: classic PKU \[blood Phe greater than {\>}1200 micromole per liter {mcmol/L}\], mild PKU \[blood Phe 600 to1200 mcmol/L\] or mild hyperphenylalaninemia (HPA) \[blood Phe 300 to 600 mcmol/L\] * Subjects who have received no previous treatment with sapropterin dihydrochloride (either Kuvan® or any other formulations of tetrahydrobiopterin \[BH4\]) * Subjects adherent to their normal diet and willing to adhere to the given diet for the 4 weeks study period * Subjects who provide a signed (by parent if below 18 years) written informed consent * Subjects with documented genotyping for both phenylalanine hydroxylase (PAH) mutations (PKU genotype) * Phenylketonuria (PKU) diagnosis should be documented with at least two historical blood Phe levels above 400 mcmol/L * Female subjects of childbearing potential (and, if appropriate, male subjects with female partners of childbearing potential) must be willing to avoid pregnancy by using an adequate method of contraception (defined as two barrier methods or one barrier method with spermicide, or intrauterine device or use of the oral female contraceptive) for 4 weeks prior to, during and 12 weeks after the last dose of trial medication * Women of childbearing potential (for the purpose of this trial, women of childbearing potential are defined as "All female subjects after puberty unless t…