IFNB1 and infection: To quantify this, A549/pr(IFN-β)GFP reporter cells (for characterization of this cell line see references 18, 34, and 35) were infected with PIV5-W3 at a multiplicity of infection (MOI) of 0.001 and at 2 days postinfection (p.i.)the cells were trypsinized, fixed, stained for NP and the number of GFP-positive (GFP +ve) cells was compared to the number of cells positive for NP by FACS analysis (Fig. 1b).