While PVM clearly replicates efficiently in mouse lung tissue, the in vitro propagation of this pathogen is significantly less straightforward. The primate BS-C-1 epithelial cell line supports minimal rates of PVM replication in vitro [10] . The BS-C-1 cell line has been used for traditional plaque assays, but PVM-induced plaques develop slowly, have relatively indistinct borders, and are difficult to eval-uate quantitatively [see Figure 1A ]. Furthermore, from an evolutionary perspective, one would prefer to perform molecular studies of virus pathogenesis in cells from a relevant species, i.e...a rodent cell type or cell line. We have demonstrated that PVM replicates in the mouse LA4 respiratory epithelial cell line [11] , but virus growth is similarly slow, even at temperatures permissive for virus propagation in vitro.