Differential effect of ND-4043 on donors and targets in HIV-1-mediated fusion (upper panel X4, lower panel R5) entry. The fusion assay was carried out in complete medium plus 2.5% of heat-inactivated FCS. Both donor and target were 3T3 cells: as donors they were infected with vaccinia virus expressing X4 (vCB41) or R5 (vCB43) HIV-1 envelope, as targets they were stably transfected with CD4 plus CXCR4/CCR5. Ctrl = control is the mixture of the donor and target populations of 3T3 cells in the absence of ND-4043. Three incubation conditions were compared in the same experiment. Viral fusion is expressed as OD and reported data are mean ± SD of three replicates. The compound ND-4043 is tested at 30, 10, 3 and 1 μM.