NiV expression plasmids pCAGGS-G, F, and M are all under the control of chicken beta actin promoter [56] , and they were constructed in the laboratory of one of the co-authors of this study (CB) as described previously [20] . Human embryonic kidney 293 cells (ATCC, CRL-1573) and 293T cells (ATCC, CRL-11268) were grown in Dulbecco's minimum essential medium supplemented with10% fetal bovine serum (FBS) and penicillin and streptomycin, and maintained in the same medium containing 2% FBS. The minigenome that was used for optimizing VLP formation has been described previously [57] . All the initial minigenome-based optimization steps were done in BHK-T7 cells (a gift from Dr. N. Ito). The same conditions were applicable to produce VLPs in 293T cells and they were used throughout to generate the VLPs used for the work described in this study.