The PB1-F2 protein of the strain A / Duck / VietNam / QT802/2011 (H5N1), and the other two strains, namely A / Chicken / VietNam / DT382/2008 (H5N1) and A / Duck / VietNam / TG926/2009 (H5N1), have a mutation of changing amino acid arginin to serin at the 66th position (N66S).