FIG. 6.

In vitro growth properties of BAC MHV-68 RγHV68A98.01, of ORF 4 deletion mutant RγHV68A98.05, and of revertant virus RγHV68A99.03. NIH3T3 cells were infected at an MOI of 0.1 for 1 h at 4°C to allow adsorption. For penetration, prewarmed medium was added for a 2-h period of incubation at 37°C. Remaining extracellular virus was inactivated by treatment with low-pH citrate buffer for 1 min. Cells and supernatants were harvested at the indicated time points, and viral titers were determined by plaque assay.