Rabbit polyclonal anti-PB1 serum V19 raised against amino acids 50-370 of PR8 PB1 has been previously described (28) , as has rabbit polyclonal antiserum A2915 against PR8 NP (29) . Rabbit antisera to the C-terminus of the PB1-F2 protein and to the full length PB1-F2 protein were kindly provided by Jonathan Yewdell. Rat monoclonal anti-tubulin YL1/2 was purchased from Serotec, anti-GFP mouse monoclonal JL8 from Clontech and IR800 or IR680 dye conjugated anti-rabbit IgG and anti-mouse IgG sera were purchased from LiCor.