FIG. 4.
Growth curves of cloned virus and parental VR-2332 isolate. Parallel cultures of Marc-145 cells were infected at a multiplicity of infection of 0.002 TCID50/cell with virus recovered from the full-length cDNA clone and the parental VR-2332 isolate. After 2 h of incubation at 37°C, the cells were washed twice and fresh medium was added (time 0), and the cells were incubated at 37°C. At 0, 3, 16, 20, 24, 48, and 72 h postinfection, samples of the supernatants were taken, and virus titers were determined as described in Materials and Methods.