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Summary: Since 2013, H7N9 avian influenza viruses (AIVs) have caused more than 1,600 human infections, posing a threat to public health. An emerging concern is whether H7N9 AIVs will cause pandemics among humans. Molecular analysis of hemagglutinin (HA), which is a critical determinant of interspecies transmission, shows that the current H7N9 AIVs are still dual-receptor tropic, indicating limited human-to-human transmission potency. Mutagenesis and structural studies reveal that a G186V substitution is sufficient for H7N9 AIVs to acquire human receptor-binding capacity, and a Q226L substitution would favor binding to both avian and human receptors only when paired with A138/V186/P221 hydrophobic residues. These data suggest a different evolutionary route of H7N9 viruses compared to other AIV-subtype HAs. : Residue substitution in hemagglutinin (HA) could be a determinant of interspecies transmission of influenza virus. Xu et al. report a unique evolutionary route behind the avian-to-human receptor-binding adaptation in H7N9-subtype HA. Keywords: influenza A virus, H7N9, hemagglutinin, receptor-binding property, interspecies transmission, structural basis