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(A) HA, (B) M, (C), NA, (D) NP, (E), NS, (F) PA, (G) PB1, and (H) PB2 segments are shown. Pre-1977 H1N1, H2N2, H3N2 isolates are indicated with blue Xs, and re-emergent H1N1 isolates are indicated with red Os.
