Cover photograph (Copyright © 2001, American Society for Microbiology. All rights reserved.): Cryoelectron micrograph of a field of immature human immunodeficiency virus. Inset, detailed interpretation of the density, with the positions of the domains indicated on the color block diagram. The matrix domain (red) appears as a thin layer tightly associated with the inner leaflet of the membrane. This domain is separated from the two subdomains of the capsid layer (green) by a low-density region. The nucleocapsid (purple) forms the innermost layer together with the genomic RNA. The envelope proteins (black) appear as projections from the outer surface of the bilayer. (See related article on page 759.)