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Phylogenetic trees of PB2, PB1, and PA genes. Trees were generated as described in the legend of Fig. 2 and in Materials and Methods. Analysis was based on nucleotides 1079 to 2138 (PB2), 42 to 1217 (PB1), and 251 to 2049 (PA). The PB2 tree was rooted to A/equine/Prague/1/1956 (H7N7), and the PB1 and PA trees were rooted to A/Brevig Mission/1/1918 (H1N1). Viruses characterized in this study are highlighted in red. Scale bar, 0.1 substitutions per site (0.01 substitutions for PA).
