Once the NC has reached the PM, two contrasting mechanisms have been proposed for filovirus budding. In the ''submarine-like'' model, budding proceeds via lateral association of the NC with the membrane, followed by horizontal budding. In the ''rocket-like'' model, the NC is extruded vertically from the PM. The analysis of budding MARV particles presented here allows the sequence of steps resulting in filamentous virus budding and release to be described in 3D. Viral NCs are assembled in the cytoplasm ( Figure 5A ) and delivered in their full-length form to the PM, with which they associate laterally, along one side, for their entire length ( Figure 5B ). Envelopment of the PM-associated NC is initiated at one end, and proceeds along the length of the NC ( Figure 5C ) until the nascent virion protrudes from the membrane, remaining attached at only one end ( Figure 5D ). Only very few particles are seen which appear to be partly extruded, whereas larger numbers of NCs are either associated with the PM prior to extrusion or are fully extruded and protruding from the PM. This strongly suggests that extrusion is a rapid process in comparison with its initiation or the subsequent membrane scission event.