Flow cytometric analysis of gp120 incorporation in the cell membrane of transfected HEK293T cells. HEK293T cells were transfected to produce WT or gp41 mutant HIV-1NL4.3. After 3 days, gp120 on the cell surface was detected using the monoclonal antibody 2G12. The percentage of gp120-expressing cells was determined using flow cytometry. Data are the means ± SEM of 2 independent experiments. The difference between WT and mutant virus was considered to be significant when the p value calculated using the student’s t-test was <0.05 (* = p < 0.05, ** = p < 0.005).