Inclusion Criteria:  Participants with PKU will be included if they were diagnosed as newborns and immediately started on treatment.  Exclusion Criteria:  Individuals with a history of neurologic compromise or major medical disorder unrelated to PKU (e.g., closed head injury, diabetes, multiple sclerosis) will be excluded. In addition, individuals with PKU will be excluded if they are on a medication called Peg-Valiant/ PEG-PAL/ Pegvaliase/Palynziq.