Given that PIV5 infection blocks STAT1-dependent signaling by IFN-γ [27], and IFN-γ has been reported to upregulate some NK cell inhibitory ligands [20,21], we examined the effect of PIV5 infection on the cell surface expression of three prototypic NK cell inhibitory ligands—PD-L1, HLA-E, and MHC-I—following IFN-γ treatment. This evidence concerns the gene STAT1 and infection.