We also observed ''empty'' filaments that lack nucleocapsids, with a random length and a smaller diameter ( Figure S1C ). These empty nucleocapsids are also visible in previously published thinsection micrographs of human EBOV infected pathology specimens, e.g. [23] , and are thus probably not an artifact of cell culture. It is also unlikely that the polyploidy we have observed in EBOV is a peculiarity of the particular viral isolate or of the Vero cell line that we used for this investigation. Previous studies using negative stain EM have shown that both Marburg virus and several different EBOV species can produce filamentous virions up to 14 mm long when grown in several different cell lines [21, 22] . Blood specimens from Guinea-pigs and monkeys that were inoculated with primary isolates of the Marburg agent, when centrifuged and observed by negative stain, also showed filamentous virions of variable length, with the average reported as about 1 mm in length but with a smaller proportion being over 2 mm in length [24] . In addition, viral filaments of lengths from 1.4 to 1.6 mm long, and occasionally as long as 2.6 mm, can also be seen in published ultrathin sections of EBOV infected human and monkey tissues [23, 25] . Thin-section EM always underestimates the lengths of virions, since filament profiles are frequently truncated by the section plane: in addition, tissue shrinkage of 10 to 20% during dehydration and resin embedding is common. Thus, it is probable that polyploid EBOV with multiple nucleocapsids are also produced in naturally infected humans and animals, though it is not possible to make a direct comparison with cell cultured virus: it would be very difficult to obtain large enough quantities of concentrated virus from animals to carry out detailed cryo-TEM analysis and measurements.