Peter A. Calabresi is Professor of Neurology, Director of the Richard T. Johnson Division of Neuroimmunology and Neuroinfectious Diseases, and Director of the Johns Hopkins Multiple Sclerosis Center, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA. Laura J. Balcer is Professor and Vice Chair, Department of Neurology, New York University Langone Medical Center, New York, USA. Elliot M. Frohman is a Professor of Neurology, Department of Neurology, University of Texas Southwestern Medical Center, Dallas, Texas, USA.