Cover photograph (Copyright © 2001, American Society for Microbiology. All rights reserved.): Human immunodeficiency virus type 1 (HIV-1) Gag and Env colocalize within virus assembly sites at the plasma membrane. COS-7 cells transfected with a noninfectious HIV-1 proviral cDNA construct were fixed, and Env protein at the cell surface was visualized by immunofluorescence with anti-Env antibody (red). The cells were then permeabilized, and Gag was visualized with anti-Gag antibody (green). Nuclei were counterstained with Hoechst (blue). The cells were analyzed by laser-scanning confocal microscopy; a single confocal slice is shown. Note the striking punctate fluorescent staining patterns of both Gag and Env, with extensive overlapping yellow areas corresponding to Gag and Env colocalization at the plasma membrane. (See related article in the September 2000 issue: vol. 74, issue 18, p. 8670-8679.)