IFNA1 and infection: In the present study, we used Vero cells (representing African green monkeys, which are susceptible to HPIV1 infection), which do not express type 1 IFNs and thus permit evaluation of IFN signaling without the confounding effects of endogenously-produced IFN, to examine at what stage in the pathway WT HPIV1 succeeds and F170S HPIV1 fails to inhibit IFN signaling.