We next used the A549/pr(IFN-β).GFP cells to monitor the activation of the IFN-β promoter in individual cells during infection by viruses generated by passage at low m.o.i. Such preparations would be typical working stocks, lacking or low in DI particles, and are generally poor inducers of IFN-β (see for example Poole et al., 2002). This evidence concerns the gene IFNB1 and infection.