IFNB1 and infection: Differences in GFP expression were quantified by FACS analysis: following infection of the A549/pr(IFN-β).GFP cells with 1–2 PFU/cell, ~ 12% of cells were positive for GFP following infection with PIV5-VΔC vM0, while ~ 80% were positive following infection with PIV5-VΔC vM2 (Fig. 5B).