Table 1.

Evaluation of genetic stability in the PB1 gene of B/Bris PB1 3M and B/Bris PB1att 4M strains after serial passage in MDCK cells and SPF eggs. Newly acquired mutations during the serial passages that were not introduced via reverse genetics are highlighted in bold. All mutations were confirmed by Sanger sequencing. sE1 and sE4 represent the 1st and 4th serial passage in SPF eggs, respectively. sP1 and sP4 represent the 1st and 4th passage in MDCK cells, respectively.

Strain	SPF Egg Passage
	sE1	sE4
B/Bris PB1 3M	K391E, E580G, S660A	E48K, K391E, E580G, S660A, G161C
B/Bris PB1att 4M	E48K, K391E, E580G, S660A	E48K, K391E, E580G, S660A
Strain	MDCK Passage
	sP1	sP4
B/Bris PB1 3M	K391E, E580G, S660A	K391E, E580G, S660A
B/Bris PB1att 4M	E48K, K391E, E580G, S660A	E48K, K391E, E580G, S660A