The main goal of this project is to identify and develop biomarkers to be used for treatment trials in presymptomatic human genetic prion disease (gPrD). Most human prion diseases (PrDs) are potentially transmissible, rapidly progressive, and all are fatal. Development of early detection methods and treatments is vital. Approximately 15% of human prion diseases (PrDs) are genetic in origin, caused by mutations in the prion protein gene (PRNP). Because a simple genetic test can identify mutation carriers (from families with gPrD) before symptom onset (presymptomatic; PreSx), this group is a logical target for therapeutic trials to delay, or prevent, the onset of disease. Therapies are currently under development, but to prepare for these trials, it is necessary to identify markers sensitive to biological changes in early stages, when symptoms have not yet developed. Our preliminary data suggest that such biological changes can be measured in PreSx gPrDs. These biomarkers include brain volume, mean diffusivity on diffusion tensor imaging, cognitive tests, motor assessments, cerebrospinal fluid (CSF) and possibly plasma proteins, and thickness of certain retinal layers. Over five years, we will recruit ~65 PreSx gPrD mutation carriers, and ~35 age and gender-matched controls (controls enrolled through separate programs at our center), who will be evaluated at least three times each at one year intervals over five years. Each annual visit will include a detailed assessment (neurological exam, neuropsychological testing, functional scores, blood and CSF collection, olfactory mucosal swabbings, brain MRI, and optical coherence tomography). The rates of change of various biomarkers will be used to create a model using the best combinations of weighted biomarkers as outcome measures in treatment trials for PreSx gPrD. Our aims are: 1. Characterize the rates of biomarker change in PreSx Slow-gPrD. We hypothesize that, compared with controls, PreSx Slow-gPrD will show: A) Greater rates of cortical volume loss, B) Greater rates of cortical MD elevation, C) Greater rates of elevation of CSF total tau (t-tau) and serum and CSF neurofilament light chain (NfL), D) Greater rates of cognitive and motor decline, and E) Greater rate of decline in INL thickness 2. Characterize the rates of biomarker change in PreSx Fast-gPrD. A) Greater rates of cortical volume loss, B) Great rates of deep nuclei (putaminal) MD reduction, C) Greater rates of elevation of CSF total tau and serum and CSF neurofilament light chain, and D) Greater rates of cognitive and motor decline.3. Determine the relationship between RT-QuIC positivity and rate of biomarker change. We hypothesize that in subjects with positive RT-QuIC assays will have more rapid changes compared with RT-QuIC negative subjects. 4. To develop optimal composite measures for tracking longitudinal change in PreSx gPrDs. Using principal component analyses and multidimensional scaling, we will identify the best weighted combinations of biomarkers for tracking PreSx Fast and PreSx Slow gPrD.