Response to Phenylketonuria to Tetrahydrobiopterin (BH4)

Inclusion Criteria: * Subject and/or parent or guardian must be capable of understanding and providing written informed consent * Subjects must have Phenylketonuria (PKU)or hyperphenylalaninemia (HPA), defined as baseline blood Phe levels of \>600 umol/L * Subjects must be at least 10 years of age, and may be of either gender and any ethnic group * Female subjects of childbearing potential must agree to use adequate birth control or refrain from sexual activity throughout study participation Exclusion Criteria: * Female subjects who are pregnant or breastfeeding * Subjects who have concurrent diseases or conditions that require medication or treatment * Subjects who require concomitant treatment with any drug known to inhibit folate synthesis * Subjects who have been treated with any investigational drug within 30 days