Although more than 80% of H9N2 influenza virus isolates from China possess a leucine at HA position 226 (http://blast.ncbi.nlm.nih.gov/Blast.cgi), the limited number of reported human H9N2 infections suggests that α2,6-linked SA receptor binding is not sufficient for infection or human-to-human transmission. Here, IGKV6-21 is linked to infection.