Hemagglutinin (HA) and neuraminidase (NA) play critical roles in overcoming this genetic host barrier, as shown by recent zoonotic infections with H7N9, H10N8, and H5Nx viruses, all containing H9N2 internal genes, compared with the remarkable dearth of reported human infections with H9N2 viruses, despite their higher incidence in poultry (15,22,23). The gene discussed is XK; the disease is infection.