The focus of neuropsychological testing will be executive abilities, as these abilities are particularly susceptible to disruption in individuals with PKU. We hypothesize that improvements in these abilities will occur following treatment with sapropterin. For neuroimaging assessments, structural magnetic resonance imaging (MRI) will permit evaluation of changes in the structure and volume of the gray and white matter of the brain, whereas diffusion tensor imaging (DTI) will permit evaluation of microstructural white matter integrity.