Elena A Zehr1, Antonina Roll-Mecak2,3. 1. Cell Biology and Biophysics Unit, National Institute of Neurological Disorders and Stroke, Bethesda, MD, USA. Elena.Zehr@nih.gov. 2. Cell Biology and Biophysics Unit, National Institute of Neurological Disorders and Stroke, Bethesda, MD, USA. Antonina@nih.gov. 3. Biochemistry and Biophysics Center, National Heart, Lung and Blood Institute, Bethesda, MD, USA. Antonina@nih.gov.