Cover photograph (Copyright © 2001, American Society for Microbiology. All rights reserved.): JC virus-associated progressive multifocal leukoencephalopathy in a human immunodeficiency virus-infected patient. This whole-brain coronal section shows extensive destruction of the right hemisphere white matter (hematoxylin and eosin, Luxol Fast Blue), with sparing of the cortical ribbon. (Special thanks to Jeffrey T. Joseph, Beth Israel Deaconess Medical Center, and Umberto De Girolami, Brigham and Women's Hospital, Boston, Mass., for histologic preparation and diagnosis.) (See related article on page 3483.)