293T cells were grown in Dulbecco's complete medium to achieve semi-confluent (80-90% density) cell monolayers. The cells were transiently transfected with the plasmids constructs using the lipid reagent Lipofectamine 2000 according to the general guidelines provided by the manufacturers' instructions (Invitrogen Inc). At 48 hrs post-transfection, the VLP-containing cell supernatants (SUP) were harvested for concentration and purification of the VLPs. Because of the fusogenic property of our VLPs, there was widespread syncytia formation at this time point although the cells were still adherent.