CD4 and infection: The absolute infection efficiency of these T/F viruses in U87.CD4.CCR5 cells, as measured by the gaussia luciferase activity, was plotted in Figure 2B. We noticed that three T/F HIV-1 strains, that is, p6240_08.TA5.4622, pBRB958_06.TB1.4305 and pBRB931_06.TC3.4930 infected U87.CD4.CCR5 cells with an efficiency comparable to or even higher than that of AD8 (Figure 2B).