1. Laboratory for Drug Discovery in Neurodegeneration, Harvard Center for Neurodegeneration and Repair, Brigham & Women's Hospital and Harvard Medical School, 65 Landsdowne Street, Cambridge, Massachusetts 02139, Department of Cell Biology, Harvard Medical School, 240 Longwood Avenue, Boston, Massachusetts 02115, and Department of Biochemistry, Tufts University Medical School, 136 Harrison Avenue, Stearns 703, Boston, Massachusetts 02111