Rac1 is activated in K1-expressing cells and in KS tumors. (A) HEK293 cells were transfected with either pcDNA3 vector alone or pcDNA3-K1 and 48 hours after transfection, cells were harvested and subjected to immunoprecipitation with anti–Rac1-GTP antibody followed by Western blotting for Rac1. Two independent anti–Rac1-GTP immunoprecipitations from vector and K1-expressing cells are shown. Expression of K1 and Rac1 in these cells was verified by Western blot. (B) Immunohistochemistry showing active Rac1-GTP in tumor sections from mice injected with KSHV-infected endothelial cells. (C) Immunohistochemistry showing active Rac1-GTP in normal human skin sections. (D) Immunohistochemistry showing active Rac1-GTP in KS tumor tissue sections. No staining was observed in the presence of a control antibody (panel B). Original magnification, ×400.