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Phylogenetic tree of influenza A HAs. The 2 groups are colored cyan (group 1) and green (group 2), each of which can be further subdivided into 3 clades (H8, H9, and H12; H1, H2, H5, and H6; H11, H13, and H16) and 2 clades (H3, H4, and H14; H7, H10, and H15). The tree was drawn by using the program TreeIllustrator (40).
