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Phylogenetic tree of HA sequences derived from human H7N9 viruses. The evolutionary history was inferred using the Neighbor-Joining method with Kimura distances. Five major clusters are shown as a collapsed branch. A/Netherlands/219/2003 is defined as an outgroup. The Yangtze River Delta and Pearl River Delta lineages are circulating in China. Highly pathogenic H7N9 viruses, which harbor multiple basic amino acids in the HA cleave site, are included in the Yangtze River Delta lineage.
