The reduced binding to sialic acid receptors as a result of the A134V mutation not only leads to changes in pseudotyping efficiency, but is also found to have an impact at the whole virus level. Reverse genetics generated RG-A/Cambodia/408008/2005 with the A134V mutation has been shown to agglutinate to the same degree both human red blood cells (RBCs), which express alpha-2,6-sialic acid, and guinea pig RBCs, which exhibit both alpha-2,3 and alpha2,6-linked sialic acid, but failed to agglutinate horse RBCs, which carry only alpha-2,3-sialic acid [30] . These observations provide an experimental evidence to support the notion that the A134V mutation leads to a reduced alpha-2,3sialic acid binding of the virus. To confirm the effect of A134V mutation on H5N1 viruses, we performed similar hemagglutination assays using another H5N1 virus strain A/Cambodia/ V0401301/2011, which also contains the same A134V mutation. 