Furthermore, whereas the WT mice infected with the WT rPICV produced high levels of early IFNβ and ISGs at day 3 post infection that were diminished by day 6 post infection (Figures 5D–G), none of the rPICV-NPmut-infected mice produced significant levels of these genes at both time points, except for a comparable level of ISG15 expression in the KO mouse lines at day 3 post infection as compared to that in the WT mice infected with the WT rPICV, the significance of which is not known. This evidence concerns the gene ISG15 and infection.