ISG15 and infection: We quantitated the induction of IFIT1, viperin, IFN-β, IFITM1, ISG15, OAS1 and Mx1 mRNA by qRT-PCR at 24 and 48 h.p.i. Early in infection (24 h.p.i) cells were particularly unresponsive to ZIKV infection even though a significant proportion of the culture was infected (Fig. 1A,B).