From the *University of California San Diego, Department of Neurosciences, La Jolla, California; †Department of Radiology, Mayo Clinic, Rochester, Minnesota; ‡Banner Good Samaritan Medical Center, University of Arizona, Translational Genomics Research Institute, Arizona Alzheimer's Disease Consortium, Phoenix, Arizona; §Department of Psychiatry, Laboratory of Molecular Neuropharmacology, Western Psychiatric Institute and Clinic, University of Pittsburgh Medical Center, Pittsburgh, Pennsylvania; ∥Center for Imaging of Neurodegenerative Diseases, VA Medical Center; ¶Department of Radiology, University of California, San Francisco; San Francisco, California; #Center for Neurologic Diseases, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts; **Institute of Clinical Neuroscience, Department of Experimental Neuroscience, Sahlgrenska University Hospital, Göteborg University, Göteborg, Sweden; ††Department of Neurosciences, University of California, San Diego; San Diego, California; ‡‡Department of Pharmacology, Center for Experimental Therapeutics, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania; §§Donald W. Reynolds Institute on Aging, Department of Geriatrics, University of Arkansas for Medical Sciences and GRECC VA Medical Center, Little Rock, Arkansas; ∥∥Elan Pharmaceuticals, South San Francisco, California; and ¶¶Eli Lilly and Company, Indianapolis, Indiana.