While MNV is an efficient enteric virus that infects many mice in research mouse colonies around the world, diarrhea has not been reported after MNV infection. Thus, MNV provides an infection only model for HuNV infection. Viral titers in tissues of infected mice have not been reported to exceed 10 6 PFU/ml, and this highest level of viral titer is obtained after infection of highly susceptible STAT1-/-mice [31] . In RAG1-/-mice and WT mice, viral titers of 10 2 to 10 4 PFU/ml are routinely observed [9, 31] . The availability of a plaque assay for MNV allows the analysis of MNV infection despite these low titers. Some MNV strains persist at a low level in WT mice, while others are cleared from intestine, spleen, liver, mesenteric lymph nodes (MLN) and feces within 7 days of infection [9, 27, 31] . Additionally, in wild type C57BL6/J mice MNV replicates maximally in the distal ileum [9] , in comparison to wild type 129S6/SvEvTac mice where replication occurs in the proximal intestine [31] . The significance of these differences is not known.