Complementation with laboratory-adapted and primary Envs from different clades. The vaccinia system was employed to assay cell fusion between effector cells expressing Envs and target cells expressing CD4 either with (filled bars) or without (open bars) the indicated coreceptor. A) CXCR4-dependent fusion. Effector cells expressed the indicated Envs either individually (top section) or in combination with LAI-FP26 (middle section). Effector cells expressed LAI-V3 individually or in combination with LAI-FP26 (bottom section). B) CCR5-dependent fusion. The indicated wt Envs were assayed for their intrinsic fusogenicity with target cells expressing CD4 and CCR5.