IFIT1 restricts viral infection posttranscriptionally by blocking the translation of viral mRNA (17, 27); therefore, we predicted that IFN-α–pretreated A549-ISG15−/− cells would remain susceptible to infection but that high levels of IFIT1 would mean these cells would not be permissive to PIV5 infection. The gene discussed is IFIT1; the disease is infection.