A cell-free infectivity assay was set up, as schematically illustrated in Figure 1C. PVs were generated by co-transfecting 293T cells with one of the S-protein-coding plasmids, HIV-1 packaging vector pCMV-dR8-2, and an improved intron-regulated reporter vector pUCHR-inLuc-mR, capable of measuring both cell-free and cell coculture infections using the mean of luciferase activity [38,39]. This evidence concerns the gene PROS1 and infection.