TABLE 1.

Conservative nucleotides and base pairs at 3′-end positions 13 to 15 and 5′-end positions 14′ to 16′

Segment and/or subtype(s)	3′-end positions 13–15	5′-end positions 14′–16′	Base pairing at the indicated positionb
			13-14′	14-15′	15-16′
PB2	AGU	UCG	√	√	×
PB1	GUU	CAU	√	√	×
PA	AUG	UAC	√	√	√
NP	CAU	GUA	√	√	√
M	AUC	UAC	√	√	×
NS	CAC	GUG	√	√	√
HA
1–16	CCXa	GUG	√	×	√ or ×
NA
1, 2, 4, 5, 8	UCA	AGU	√	√	√
6, 7	CAC	GUG	√	√	√
3	ACG	UGC	√	√	√
9	CAG	GUC	√	√	√

^aX represents either A or U at position 15 in the 3′ end that is variable among different HA subtype segments.

^bThe presence (√) or absence (×) of base pairing is indicated.