SARS immunohistochemistry (IHC) on lung sections shows diffuse strong SARS positivity in pneumocytes in >50 cells per 4 mm2 (a, b). TTF-1/SARS double labeling (c) demonstrates co-localization of TTF-1 (brown reaction product) and SARS (red reaction product) in pneumocytes (c, arrow). PU.1/SARS double labeling demonstrates expression of PU.1 (brown reaction product) in macrophages, most of which did not stain for SARS (red reaction product), and SARS mostly in pneumocytes negative for PU.1 (d). Weak staining for SARS was identified only in scattered macrophages (d, arrow). SARS IHC on cross sections of trachea (e) demonstrates positive staining predominantly in ciliated cells (e, arrows). P63/SARS double labeling demonstrates nuclear expression of p63 in basal cells (brown reaction product) (f, arrow) which did not stain for SARS (red reaction product). MUC5AC/SARS double labeling shows cytoplasmic expression of MUC5AC in secretory cells (brown reaction product) (g, arrow) most of which did not stain for SARS (red reaction product). FOXJ1 highlighted uninfected ciliated cells (h, arrowhead) but was absent in SARS-expressing cells (h, arrow).