Previous studies with MHV68 mutants have shown that muSOX is critical for replication in cultured cells [16, 19] . To determine the contribution of host shutoff activity towards this defect, we performed a multi-step growth curve in murine fibroblasts. 3T3 cells were infected at a multiplicity of infection (MOI) of 0.1 plaque forming units (pfu) per cell and MHV68.WT, MHV68.DHS, and MHV68.MR all replicated to equivalent titers and with similar kinetics (Figure 4) . We sequenced the muSOX gene from MHV68.DHS samples at 5 days post-infection (dpi) and found no evidence for reversion or secondary mutations (data not shown). Therefore, muSOX-dependent mRNA degradation had little effect on viral titers in 3T3 cells through multiple replication cycles.