Env-mediated cell-cell fusion. COS-1 cells transiently transfected with the Env expression vector were cultured for 24 h. The COS-1 cells transiently expressing WT and mutant glycoproteins were co-cultured with TZM-bl indicator cells. Following a 24 h incubation, the co-culture of cells was lysed and measured for luciferase activity. P values were calculated by using Tukey's T-test and a value <0.001 are shown with an asterisk. The data represents results from three independent experiments conducted in triplicate. In these assays WT fusion yielded an average of 4.7 × 105 DLU and EBFP an average of 8.4 × 103 DLU. The error bars represent the standard deviation of the means.