By Vinay Kumar, MBBS, MD, FRCPath, Professor and Chairman, Department of Pathology, University of Chicago, Pritzker School of Medicine, Chicago, IL; Abul K. Abbas, MBBS, Emeritus Professor, Department of Pathology, University of California San Francisco; Jon C. Aster, MD, PhD, Professor, Department of Pathology, Harvard Medical School; Brigham and Women's Hospital, Boston, Massachusetts and Andrea T Deyrup, M.D., Ph.D., Associate Professor Department of Pathology Duke University Medical Center