COS-1 cells transiently expressing OA1-GFP derivatives were fixed with methanol and were incubated with monoclonal anti-CD63 antibody H5C6 followed by the labeling with Texas Red conjugated goat antimouse IgG. The cells were visualized by fluorescence microscopy with the fluorescence from fused GFP (A, D, and G) and anti-CD63 (B, E, and H). The merged images (C, F, and I) indicate the colocalization of OA1-GFP derivatives and CD63, the marker for late endosome/lysosome, in perinuclear granules. OA1-GFP derivatives expressed in COS-1 cells were: wild-type OA1-GFP (A, B, and C); OA1-GFP (L3) (D, E, and F); and OA1-GFP (L6) (G, H, and I). The other derivatives that are not shown here were similarly colocalized with CD63.