We next verified that the R443I mutation in MHV68.DHS conferred a host shutoff defect during viral infection. We assayed levels of several endogenous mRNAs by real-time quantitative PCR (RT-qPCR) following infection of NIH 3T3 fibroblasts with wildtype MHV68 (MHV68.WT), MHV68.DHS, or MHV68.MR. WT