Recombinant isogenic DM strain of MHV (RSA59) and NDM strain (RSMHV2) have been described in previous studies (4, 9) . RSA59 and RSMHV2 strains of MHV are isogenic except for the spike gene, which encodes an envelope glycoprotein that mediates many biological properties of MHV including viral attachment to host cells and virus-cell and cell-cell fusion (10) . These recombinant strains also express enhanced green fluorescence protein (EGFP) (4, 9) .