Infectivity of glass-bound VLPs. MLV-based vector particles carrying the β-galactosidase marker gene and the indicated Env proteins were purified from the supernatants of transfected 293T cells. Comparable amounts of particles (as determined by anti-MLV CA immunoblot) were adhered to fibronectin-coated coverslips, and DFJ-8 cells were allowed to settle on top of the VLP coated surface. (A) Following 48 hours of incubation at 37°C, cells were fixed and stained for β-galactosidase activity. (B) Infected cells were counted in 5 fields of view each (corresponding to ~500 cells) per experiment. The graph shows mean values and standard deviations from three independent experiments.