Visualization of a fusion event in real time. (A) MA.mCherry and Env.YFP double labeled particles were immobilized onto a fibronectin coated cover slip, and DFJ-8 cells were allowed to settle on the particle layer. Image acquisition with a frame rate of 0.76 frames/sec was started as soon as the first cells reached the microscope slide (~1 minute after cell addition; see Additional files 5 and 6). Still images taken from the movie shown in Additional file 5 at the indicated time points after the start of image acquisition are shown. The particle of interest is indicated by a white circle. Scale bar = 10 μm. (B) Plots of fluorescence as a function of time. Depicted are normalized intensity values of the Env.YFP signal (green) and the MA.mCherry signal (red) of the virus particle monitored in (A) (indicated by a white circle) and the background intensities of the Env.YFP channel (grey). Time indicates the duration of virus-cell contact in seconds.