Subjects will undergo screening evaluations to determine eligibility prior to heavy water (2H20) administration. Eligible subjects will be admitted to the clinical facility on Day -1. On Day 1, subjects will ingest small doses of 2H20 during their inpatient stay. They will also drink 2H20 for 7 more days. Subjects will undergo two lumbar punctures (LPs) for CSF samples. Subjects will return to the study site approximately 7 days after the last LP (or early termination) for a follow-up assessment and discharge.