The HA of the chicken H6N1 virus we applied possesses the GQRSRI sequence, which corresponds to the 225 to 230 positions in H3 HA [32] . We showed that most MAA + cells, but few SNA + cells, were permissive to infection by the H6N1 virus. In addition, at a low MOI, most H6N1 proteins were detected in the SAa2-3Gal-rich ciliated cells, and few in the goblet and basal cells. These results indicate that the chicken H6N1 virus, even with a Ser228 (using H3 HA numbering) in its HA, still possesses a SAa2-3Gal preference. Similarly, the HA 226 residue, critical to cell tropism in human H3 and H9N2, has been shown to be an insufficient binding site for determining the receptor tropism of the human H1, H2, H5 and the avian H6, H9 and H5N1 viruses [4, 33, 34, 35] . These results emphasize that receptor tropism is determined by several critical residues in the HA proteins, but these sites in HA are not highly conserved among different influenza viruses.