Trapping efficiency of IRES/wPRE modified HIV-based (LT) and MLV-based (RT) promoter traps. (A) Schematic of the modified trapping cassettes; legend as in Figure 1A. IRES indicates internal ribosome entry site; wPRE, woodchuck post-transcriptional regulatory element. (B) Histogram showing the trapping efficiency of the indicated vector traps. Bars indicate the percentage of integrations that express the reporter gene (trapping efficiency, mean ± SEM, n = 4). Statistical significance by Student t test of the difference between the indicated experimental groups is shown. For each trap, the mean fluorescence intensity (MFI) of GFP-positive cells (calculated from cell cultures with frequency of GFP+ cells≤ 15%) is indicated (mean, n = 2).