Cover photograph (Copyright © 2006, American Society for Microbiology. All Rights Reserved.): Three-dimensional reconstruction of porcine endogenous retrovirus (PERV)-infected murine liver. High-resolution serial z-section confocal microscopy and Imaris software were used to reconstruct three-dimensional images of mouse liver from human PERV receptor (HuPAR-2) transgenic mice that were productively infected for 8 weeks with PERV. Green represents staining with a PERV-specific anti-Gag monoclonal antibody, red represents membrane staining with BODIPY phalloidin, and blue represents nuclei stained with 4′,6′-diamidino-2-phenylindole (DAPI). The Gag staining of hepatocytes is evident, particularly at the cell membranes. (Yuri Martina is gratefully acknowledged for his work in creating this image.) (See related article on page 3135.)