IFNA1 and infection: To isolate individual virus mutants potentially affected in their activation of the IFN response, single cells scored as positive for GFP expression after infection of A549/pr(IFN-β).GFP cells with the passage 10 V1 to V6 viruses at a low MOI (0.04 PFU/cell) were plated individually onto microcultures of MDCK-V2 cells, and the supernatants of cells showing a cytopathic effect were collected.