Induction of the IFN-independent ISG viperin during HCMV infection is known to be induced by either IRF3 or IRF1, binding directly to its promoter [20,32,46,48], and this may also be the mechanism of IFIT1, IFIT2, IFIT3, CXCL10, Mx1, Mx2 and ISG15 upregulation. Here, ISG15 is linked to cytomegalovirus infection.