* dN/dS is the ratio of nonsynonymous vs. synonymous substitutions calculated by SLAC likelihood analysis.
a The internal genes of H1N1 viruses were not included due to the few sequences available (Na). H3 n = 204, N2 n = 166, internal genes n = 15, H1 n = 27, N1 n = 30.
b dN/dS for PB1-F2 was not included because the method used was biased by the PB1-F2 +1 reading frame from PB1. Synonymous substitutions in PB1 would therefore be assigned as non-synonymous in PB1-F2 not reflecting positive selection on the PB1-F2 itself.