After a prolonged period of infection, the released MARV particles have lower specific infectivity and the majority is roughly spherical in form. Like the filamentous virions, the spherical particles contain a single full-length NC, which is kinked in a number of places, but not broken, suggesting that these particles still contain a full-length viral genome. The NC is not tightly wrapped on all sides as it is in filamentous virions, but is associated with the membrane along one side for its entire length. This lateral association is also seen for intracellular NCs prior to extrusion from the PM. Since formation of spherical particles is paralleled by convolution of the PM and shedding of cellular vesicular material into the supernatant, we propose that spherical viruses are released by large-scale membrane instability at sites of budding, leading to vesiculation of viruses prior to extrusion. In this model, kinking of the NC might be induced by the forces which lead to invagination and vesiculation of the PM, though other factors could contribute, such as lack of a viral or cellular factor which rigidifies the NC. The interplay between the budding process and the dynamics or composition of the cellular membrane, as well as possible changes in the relative rates of the different steps of the budding process in different virus strains [44] , mutants [45] , cell types [46] or stages of infection, may also contribute to the variable morphology observed in other viruses.