Structural protein expression and phenotypic properties of the stem deletion mutant viruses. (A) Intracellular N protein expression of WT and mutant viruses. BHK-21 cells were transfected with plasmids of WT and mutants D1-D6 as indicated in Figure 3. Expression of N protein was visualized by immunofluorescence staining at 24 hours post-transfection. (B) Viral plaque morphology and growth curves of the mutant viruses D1-D6 as described in Figure 3.