To test the possibility that MHV mRNAs synthesized early in infection are susceptible to NMD, we inoculated MHV into cells treated with control siRNAs (NMD-competent) or with siRNAs for UPF1 or UPF2 (NMD-deficient) and examined the levels of viral mRNAs at different times p.i. (Fig. 7 B and C). This evidence concerns the gene UPF1 and infection.