TABLE 1.

Frequency of amino acid differences in the region flanking the M1_58–66 epitope^a

^a“Position” refers to the position in the M1 amino acid sequence; “no.” indicates the absolute number of viruses. Boldface indicates amino acids present in the avian H5N1 (A/Vietnam/1194/04) and human H3N2 (A/Netherlands/018/1994) viruses used in the present study.

^b All amino acid sequences present in the National Center for Biotechnology Information (NCBI; http://www.ncbi.nlm.nih.gov/genomes/FLU) database as of 11 April 2015. Yr, years of isolation that were present in the database.

^c The 95K mutation was introduced in 1997 and rapidly fixated in human seasonal A/H3N2 viruses in the following years.

^d Only one 1918 sequence (A/BrevigMission/1/1918) could be obtained from the NCBI database, which was the only virus in this group containing the 115V residue; the other H1N1 viruses in this group were from the 1930s.