The pseudo-infection model recapitulates two fundamental steps of the endosomal escape processmixing of lipid compartments and transfer of viral contents (3) (Figure ). While lipid mixing between virus and vesicle membranes is a rapid way to test that the viral fusion protein is functional , many such events are halted at the intermediate hemifused state where the outer leaflets of the two membranes merge but not the inner leaflets, and consequently, no fusion pore is formed. On the other hand, a content mixing experiment provides more information, namely, the presence of a functional fusion protein, formation of a fusion pore, and possible presence of the viral genome, though the fusion pore may not necessarily be large enough to accommodate passage of the vRNPs (viral genome).