Figure 6.

Complementation requires coexpression of Env mutants within the same cell. 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) CXCR4. Top section: Effecter cells expressing the indicated Envs individually. Bottom section: Effector cells either cotransfected with the indicated Envs, or infected separated and mixed 1:1.