Enhancement of viral replication by SIVsmm nef alleles. (A) Replication kinetics of recombinant NL4-3 variants containing the indicated nef alleles in PBMCs are shown. Infections were performed using virus stocks containing 1 ng p24 antigen. The numbers of virally infected GFP+ cells was measured by flow cytometric analysis. Similar results were obtained in two independent experiments (B) Cumulative numbers of HIV-1-infected GFP+ cells detected in PBMC cultures at days 3, 5 and 7 post-infection. Shown are averages ± SD derived from three independent infections. All nef alleles significantly (P < 0.05) enhanced the number of HIV-1-infected cells compared to the nef-defective control virus.