In preliminary studies it was found that co-expression of NiV G, F and M proteins in 293T cells resulted in the formation of VLPs that bud out into the transfected cell SUP and that they can be harvested, concentrated and purified as described under Methods. However, the VLP yield was low. To improve the efficiency of VLP formation we proceeded to optimize the ratio of the three expression plasmids used at transfection. We speculated that this would be important based on the fact that a), during replication, paramyxoviruses form a transcription gradient where the 39 proximal genes are transcribed more abundantly than the successive downstream genes [52] and b), the stoichiometry of interaction of the viral proteins has proved to be critical in plasmid-driven minigenome and full-length rescue systems [62] .