USP18 and viral infectious disease: Quantitative analysis of NP expression levels showed that viral replication was reduced by more than 95% in IFN‐α‐pretreated A549‐USP18−/− and USP18.I60N‐expressing cells, whereas the levels of viral infection in cell lines expressing the wt and the catalytically inactive mutant USP18.C64S were similar to NC1 control, confirming regulation in these cells (Figure 6C).