HEK 293T cells were seeded in 96-wells plates at a density of 10,000 cells per well and incubated overnight. For the transfection assay, cells were transfected with reporter plasmids encoding firefly or Gaussia luciferase along with expression plasmids encoding PB2, PB1, PA, or NP using Lipofectamine 2000 (Invitrogen) according to the manufacturer's protocols. Fifty nanogram of each plasmid was used in the transfection unless mentioned otherwise. For the infection assay, cells were transfected with reporter plasmids encoding firefly or Gaussia luciferase. The next day, cells were infected with IAV-WSN at a multiplicity of infection (MOI) of 1 TCID50 units per cells.