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Complementation between Env variants for CXCR4-dependent fusion. Effector cells expressing the indicated Envs (alone or in combination) were mixed with target cells expressing CD4 and either CXCR4 (filled bars) or no coreceptor (open bars). Cell fusion was quantitated by measurement of β-galactosidase activity (β-gal) after 2.5 h. Error bars indicate the standard errors of the mean of duplicate samples. wt, Wild type.
