Prevalence of amino acid residues PA-356K and PA-356R in seasonal human H1N1 and avian H9N2 influenza viruses (A) and prevalence of PA-356R alone or PA-356R/PB2-627K in H7N9 and H10N8 influenza viruses during 2013 to 2014 (B). For each column from left to right, the actual numbers of viruses are 22, 21, 1,320, 138, 590, 84, 101, 241, and 29, respectively (A) and 399, 77, 399, 77, 54, 2, 54, and 2, respectively (B). In panel A, the proportion of viruses with PA-356R (0.33%) or other residues (0.30%) is too low to be seen between 2001 and 2010. In panel B, “Others” refers to viruses without the two residues PA-356R and PB2-627K; the number of influenza viruses with PA-356R/PB2-627K is 2 for avian H7N9 viruses.