ISG15 and infection: Because paramyxoviruses (like most wild-type viruses) are poor inducers of the IFN response (28, 29), are able to efficiently and rapidly counteract it if it were induced, and our data showed that basal ISG expression was not effected in ISG15-deficient cells (Fig. 1C), we predicted that infection of naive A549-ISG15−/− cells would be equivalent to naive A549 cells.