TABLE 3 .

Amino acids at key codons in the IAV genome

Host	Virus, subtype, or location	PB2a	PA-Xb	NAc	M2d
Canine	CIV-H1N1	E627	Truncated	H275	N31
	CIV-H3N2	E627	Truncated	H275	S31
	CIV-H3N8	E627	Truncated	H275	S31
Equine	EIV H7N7	E627	Complete	H275	S31
	EIV H3N8	E627	Complete	H275	S31
Swine	CswH1	K627	Complete/Truncated	H275	S31
	EAswH1	E627	Complete	H275	N31
	TRswH3	E627	Truncated	H275	S31
	PDMswH1	E627	Truncated	H275	N31
Human	H1N1 (seasonal)	K627	Complete	H275 (1918–2006)	S31
				Y275 (2007–2009)
	H1N1pdm	E627	Truncated	H275/Y275	N31
	H2N2	K627	Complete	H275	S31
	H3N2	K627	Complete	H275	S31 (1968–2002)
					N31 (2003–2017)
Avian	Eurasian	E627	Complete	H275	S31/N31
	North American	E627	Complete	H275	S31

^aThe E627K substitution in the PB2 polymerase protein increases replication efficiency in mammals (35).

^bPA-X is a fusion protein that modulates host cellular immune responses (63). The protein is encoded by a +1 frameshift open reading frame (ORF), and a synonymous mutation in the PA gene produces a nonsense mutation at PA-X codon 42 (34).

^cThe H274Y substitution (H275Y N1 numbering) in the NA protein is associated with reduced susceptibility to oseltamivir antivirals (64).

^dThe S31N substitution in the M2 ion channel protein is associated with reduced susceptibility to adamantane antivirals (65).