Figure 4. Immunogold and in situ hybridization (ISH) labeling of severe acute respiratory syndrome–associated coronavirus–infected cells. A) Cytoplasmic area that is relatively free of organelles (arrow). B) At higher magnification, these regions are shown to consist of ribosomelike and filamentous structures. Within these regions, C) viral proteins are detected by immunolabeling, using hyperimmune mouse ascitic fluid (12 nm gold), and D) ultrastructural ISH detects viral mRNA, genRNA, or both, by using a pool of riboprobes (6 nm gold). Bars, A,1 μm; B–D, 100 nm.