The block at entry was indicated by i) different dynamics of HIV-GFP association with HFA and CD4-positive cells at 37°C as visualized by fluorescence imaging; ii) the similarity between GFP-Vpr tagged NL4-3 and entry-defective 517B viruses in their respective target cell associations by the same method; and iii) the demonstration at a single cell level that circumventing restricted HIV-1 entry by use of VSV-G pseudotyped virus permits efficient infection of HFA, as also shown by cell population analyses in our previous studies [33,34,66] and consistent with other reports [34,67,84,90]. This evidence concerns the gene CD4 and infection.