Typically, pretreatment of naive cells with IFN-α reduced infection, as shown by a reduction in NP levels, compared with nontreated cells (Figs. 2B, 2C, 3); nevertheless, because PIV5 expresses the IFN antagonist V protein, NP levels reach similar levels to untreated cells by 48 h p.i. However, this IFN-dependent reduction in virus infection is diminished when IFIT1 is knocked down, confirming earlier reports of IFIT1’s antiviral activity against PIV5 (17, 27). Here, IFIT1 is linked to viral infectious disease.