IFIT1 and infection: 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).