Table 2. Genetic stability of PB2-KO virus.

Experiment no.	No. GFP-positive plaques* from viruses in:
	Passage 1	Passage 2	Passage 3	Passage 4	Passage 5
1	10	9	9	10	8
2	10	10	9	10	9
3	10	9	9	10	9

^*The respective virus supernatants were subjected to standard virus plaque assays in confluent AX4/PB2 cells. Ten plaques were marked per well, which were then subjected to an immunodetection assay using an anti-GFP antibody to detect GFP-expressing virus plaques. The number of plaques that stained brown is presented. The results of three independent experiments are shown.