Assessment of the viral entry ability of HCV pseudotypes. Cell-free, env-defective NL4-3R-E-Luc reporter viruses produced from cotransfection with pcDNA3 (marked as the control) or with WT or mutant E1E2 plasmids were normalized for RT activity prior to challenge with Huh7 cells. Two days after infection, cells were assayed for luciferase activity, and the relative viral entry ability mediated by mutant proteins was expressed as a percentage of that mediated by WT E1E2. Results from three independent experiments are shown as the mean ± standard deviation (A). The means of the actual luciferase activities of the control and WT samples with standard deviations are also shown in (B).