N362 exerts a strain-dependent enhancement of fusogenicity by R5 Envs. Western blot analysis of 293T cells transfected with wild type and Env mutants (A). Fusion assays were performed using 293T effector cells expressing equivalent levels of wild type or Env mutants generated from control R5 ADA, YU2 and JRCSF Envs (B) or from A-R5 NB2-C4, NB6-C3, NB7-C1 and NB8-C4 Envs (C), and Cf2-Luc target cells expressing CD4 and CCR5 as described in the Methods. Cells were harvested at 2, 4, 6, 8, 10 and 12 h post-mixing and assayed for luciferase activity. Results are expressed as the percentage of maximal fusion levels attained by the wild type Env clone. Means values of duplicate infections are shown. Error bars represent standard deviations. The results are representative of 2 independent experiments. P values were calculated with a paired t-test, and values < 0.05 were considered significant*. P values approaching significance are also indicated.