The virus that caused the pandemic contained gene segments from three distinct swine influenza lineages: five of the six gene segments (PB2, PB1, PA, NP, and NS) from the triple-reassortment North American swine lineage, the HA from a classical H1N1 swine lineage, and the M and NA segments from a Eurasian avian-like H1N1 swine influenza lineage [24,25,26]. The gene discussed is XK; the disease is influenza.