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Complementation with Envs containing alterations in the membrane-spanning domain, the TRR, and HR1. 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. Within each section, effector cells expressed the indicated Envs either individually (top section) or in combination with the nonfunctional LAV-BS (lower section).
