Fig. 4From: Identifying novel amino acid substitutions of hemagglutinin involved in virulence enhancement in H7N9 virus strainsInfectivity of recombinant H7N9 virus. (a) Quantification of viral infectivity by measuring luciferase activity; (b) Quantitative detection of the interaction between recombinant HA and 3′SL- or 6′SL-labeled receptors by measuring KD valuesBack to article page