Fig. 3. Transcriptional silencing in contagious apoptosis, as determined by trans-complementation between a transactivator (Tat) and a promoter (LTR upstream of the ß-galactosidase gene) in syncytia. HeLa Env cells expressing Tat and HeLa CD4 cells transfected with the ß-galactosidase gene under the LTR promoter were co-cultured, followed by an in situ histochemical detection of ß-Gal using X-Gal as a substrate (A) or by enzymatic methods applied to cell extracts (B). Results (mean ± s.d. of triplicate samples) are representati ve of three independent experiments. #P<0.005.