We therefore inoculated U87.CD4.CCR5 cells with HIV-1-VSV G pseudotyped particles, reasoning that the highly fusogenic nature of VSV G will normalize the cellular entry of cell-free WT and revertant viruses in the first 24 hours of infection, thereby enabling an assessment of virus production following multiple rounds of cell-cell and cell-free viral transmission. The gene discussed is CD4; the disease is infection.