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Cartoon structures of the spikes of hemagglutinin (HA) and neuraminidase (NA). (a) The HA spike is formed by an HA trimer. Each HA monomer contains two functional domains, the receptor binding globular domain and the helix-rich stalk domain. (b) NA exists as a tetramer of four identical monomers. Each NA monomer consists of four distinct structure domains, including the catalytic head, the stalk, the transmembrane region and the cytoplasmic tail.
