TABLE 3.

PB2 mutations selected in recombinant viruses during mouse lung infection

Virus	No. of days p.i.	Mouse no.	Amino acid at the indicated position of the PB2 genea		Virus titer in mouse lungs (PFU/ml)
			627	701
MA-PB2(627E 701D)	1	1	E	D	6 × 103
		2	E	D	3 × 102
		3	E	D	5 × 103
	3	1	E	N	1.6 × 106
		2	E	D	1 × 105
		3	E	D	7 × 104
	5	1	E	D/N	7.5 × 104
		2	E/K	D/N	5.5 × 105
		3	K/E	N/D	2.4 × 106
	7	1	K/E	N	1.6 × 106
		2	K/E	D	5.1 × 105
		3	ND	ND	Under detection limit
MA-PB2(627E 701N)	1	1	E	N	1.9 × 107
		2	E	N	1.5 × 107
		3	E	N	4 × 108
	3	1	E	N	3.9 × 106
		2	E	N	4.2 × 106
		3	ND	ND	2.5 × 106
	5	1	E	N	3.4 × 106
		2	E	N	2.8 × 106
	7	1	E	N	1.3 × 105
		2	E	N	4 × 103

^aND, not done.