The budding of filamentous MARV therefore proceeds via an NC that is laterally associated with the PM, and a vertically protruding bud. Any changes in the rates of individual steps in the budding process could dramatically alter the appearance of the budding structures in EM. For example, if the rate of scission were significantly increased, or the rate at which extrusion is initiated were dramatically decreased, then large numbers of NCs might be expected to collect horizontally under the PM, giving the appearance of a predominant ''submarine'' mode of budding [36] . If the rate of initiation of extrusion were significantly increased, or the rate of scission decreased (for example by inhibiting recruitment or function of the cellular endosomal protein sorting system), then large numbers of protruding buds would accumulate, giving the appearance of a predominant ''rocket-like'' mode of budding. We suggest the contrasting appearance of Ebola virus and MARV infected cells in EM does not reflect different budding mechanisms, but rather different rates for the individual steps of the process presented here. These rates are likely to be both virus and cell type dependent.