Examination of membrane permeability during membrane fusion using antibodies as probes. Staining of 293CD4 cells cotransfected with the Env expression vector and pKcTac-FLAG in the presence (+) or absence (-) of C34 by anti-FLAG antibody. The pKcTac-FLAG vector expresses the Tac proteins tagged with FLAG at their C-termini. Anti-FLAG: images after staining with the anti-FLAG antibody as described in Methods are shown in green. The time points for the assay were same as Fig. 7 (16 h post transfection for wild type MSDs and 44 h for CD22MSDs, respectively). PI: Images after propidium iodide staining. Merge: merged images of Anti-FLAG and PI stainings together with that of bright fields. The concentration of C34 was 2 μM. The names of the expression vectors used are shown in left. The bar indicates 40 μm.