The 3D data also allowed us to closely examine both ends of reconstructed filamentous virions. This revealed morphological differences between the two virus ends: 19 out of 21 3Dreconstructed filamentous virions displayed a membrane bulge or hook at one end ( Figure 3A ), while the membrane formed a round, intact hemisphere at the opposite tip ( Figure 3B and Table 2 ). Only 2 out of 21 filamentous viruses had two round, intact tips. Virions with membrane distortions at both ends were never observed. Although we cannot exclude that such membrane distortions are due to chemical fixation, they suggest that a local instability of the viral membrane is specifically induced on one virus end. Thus, we made use of the fact that cells and extracellular viruses were fixed and examined in their in situ orientation. Measurement of the average distance of intact and distorted virus ends from the nearest cellular membrane revealed that intact virus ends were predominantly found at distances .250 nm away from a cellular membrane, whereas distorted virus ends were more frequently localized within 50 nm distance ( Figure 3C ). This suggests that the membrane distortions are found at the rear end of filamentous viruses where scission took place (also shown in Figure 2G and Video S1).