TABLE 1.

Critical amino acids for the receptor binding specificity of the influenza HA^a

Viral HA	Amino acid position (H3 numbering)
	77	138	186	190	194	225
A/South Carolina/1/1918	D	A	P	D	L	D
A/New York/1/1918	D	A	P	D	L	G
Avian H1 consensus	D	A	P	E	L	G

^aThe avian consensus sequence of the H1 HA was determined by comparing human and avian HA sequences (14). The above amino acids are conserved in most avian H1 HAs. Boldface type indicates a change from the A/South Carolina/1/18 HA sequence.