Viral fusion experiments. Virus particles (labeled or unlabeled) were added to the flow cell and allowed to bind to the tethered vesicles for 15 min. Unbound virus particles were removed by flowing in vesicle buffer. pH was dropped by adding fusion buffer (pH 5.1) and a continuous video stream was acquired over 1200 frames at 3.47 fps. In the event of a lipid mixing or content mixing, signal from selfquenched Texas Red-DHPE or sulforhodamine B respectively turned brighter. The wait times between lowering of pH and appearance of a bright spot were calculated and compiled into cumulative distribution functions.