NiV M, F, G and N protein-containing VLPs consistent in size and morphology to the parental virus have also been reported in a previous study which evaluated protein-protein interaction that facilitate VLP formation [48] . However, in that study, most of the particle-incorporated NiV F protein was predominantly in the uncleaved precursor form. This finding is clearly distinct from ours since our VLPs contained substantial amounts of cleaved F protein, and this may have been related to ratios of the interacting proteins expressed in 293T transfected cells. In a recent study of NDV VLPs [49] , the particle-incorporated proteins were reported to have virus-like protein ratios, but the F protein remained in its precursor form because the cleavage site required to produce the fusion competent form was mutated by design. What effect a VLP-incorporated non-fusogenic F protein may have, relative to the fusogenic form, on the level and quality of VLP-induced immune response is not clear at present since difference in immunogenicity between fusion-competent and fusion-defective VLPs has not been experimentally evaluated so far. However, a recent report suggests that viral fusogenic membrane glycoproteins may enhance vaccine potency [68] .