Immunogold labeling of our unfixed NiV VLPs confirmed that the surface proteins in our VLPs were functionally assembled and they were biologically active ( Figure 2E ). We could deduce the presence of biologically active G and F proteins on the VLP surface by the fact that they were able to induce the formation of syncytia in 293 cells ( Figure 4 ); this is a process that requires the interaction of both the surface glycoproteins, the attachment protein G, and the fusion competent F protein, when they come in contact with the cognate receptor-bearing cells. Formation of syncytia or multinucleated cells in replication competent enveloped viruses, especially paramyxoviruses, is induced by a process that is described as ''fusion from within'', and it can be blocked or neutralized by prior treatment of the virus with specific antisera. In contrast, ''fusion from without'' is induced by non-replicating viruses at high multiplicities of infection, and it too can be blocked by pretreatment with virus-specific antibodies ( [69] , and references therein; [70] ). Our non-replicating particles likewise induced syncytia formation in 293 cells that could be neutralized with NiVspecific antibodies (Figure 4 ). To our knowledge, this is the first study describing fusion from without induced by VLPs of any paramyxovirus, or any other enveloped viruses, although it has been described for the VLPs of the non-enveloped rotavirus [71] . The mechanism(s) of fusion from without is not clear but two models have been proposed [72, 73] ; one proposes that particles connecting adjacent cells effectively promote fusion between them, and the other is that when particles decorated with the surface glycoproteins fuse with the target cell membrane, the glycoprotein complexes diffuse freely in the lipid bilayer, and mimic fusion from within. The type of VLP-induced syncytia formation and eventual cell death is also not known. We are in the process of investigating it.