Frequencies of the PA-X mutations 195K and 195R in different influenza viruses. (A) The actual numbers of PA-X sequences of avian influenza viruses are as follows: 1994 to 2000, n = 83; 2001 to 2008, n = 303; 2009 to 2012, n = 422; 2013 to 2015, n = 421; 2016 to 2018, n = 189. (B) The numbers of PA-X sequences in avian H7N9 influenza viruses were as follows: wave 1, n = 54; wave 2, n = 346; wave 3, n = 264; wave 4, n = 244; wave 5, n = 105. The actual numbers of PA-X sequences in human H7N9 influenza viruses were as follows: wave 1, n = 94; wave 2, n = 207; wave 3, n = 127; wave 4, n = 118; wave 5, n = 356. (C) The numbers of PA-X sequences in H5N6 influenza viruses were as follows: avian H5N6, n = 292; human H5N6, n = 31. (D) The numbers of PA-X sequences in SIVs with the North American avian PA gene and H1N1/2009 were 24 and 183, respectively.