Kinetics of Env(wt)- and Env(NYP)-induced fusion with HeLa-CD4 cells expressing CCR5(wt) (A) or CCR5(Δ18) (B). (A) Fusion with CCR5(wt)-expressing cells induced by Env(wt) and Env(NYP) is shown by filled circles and filled squares, respectively. Fusion after establishing the 18°-TAS (pre-incubation for 2 hr at 18°C) is shown by open circles (Env(wt)) and open squares (Env(NYP)). (B) Fusion with CCR5(Δ18)-expressing cells induced by Env(wt) and by Env(NYP) is shown by semifilled circles and filled triangles, respectively. The kinetics of fusion between Env(NYP)- and CCR5(Δ18)-expressing cells after establishing the 27°-TAS (pre-incubation for 2 hr at 27°C) is shown by open triangles. The experimental points are means ± SE.