Interestingly, we noticed that plaques derived from MHV68.DHS infections differed morphologically from those obtained upon MHV68.WT or MHV68.MR infections ( Figure 5A ). To quantify these differences, we measured 75 plaques from 5 independent MHV68.DHS and MHV68.MR infections and found that, indeed, MHV68.DHS plaques were generally larger (p-value ,0.01) and had a broader frequency distribution ( Figure 5B ). This seemed unlikely to be a consequence of more rapid lytic replication, given the above growth curve results. In addition, the altered plaque size does not appear to be caused by enhanced cell-to-cell spread of the mutant virus, as we observed no significant differences in the ratio of extracellular to intracellular virus produced in 3T3 cells infected with MHV68.WT, MHV68.DHS, and MHV68.MR (data not shown).