Vinay Kumar, MBBS, MD, FRCPath, Donald N. Pritzker Professor and Chairman, Department of Pathology, Biologic Sciences Division and Pritzker School of Medicine, The University of Chicago, Chicago, Illinois, Abul K. Abbas, MBBS, Distinguished Professor and Chair, Department of Pathology, University of California San Francisco, San Francisco, California and Nelson Fausto, MD, Professor and Chair, Department of Pathology, University of Washington, Seattle, Washington