Spliced and unspliced viral mRNA exported into the cytoplasm are translated by the host cell ribosomal translation machinery (reviewed in Kash, 2006). At least ten viral proteins are synthesized: HA, NA, PB1, PB2, PA, NP, NS1, NEP/NS2 (from spliced NS mRNA), M1, and M2 (from spliced M mRNA). The abundance of each of these proteins is thought to be controlled by differential mRNA abundances and stability (Tekamp, 1980; Hatada, 1989). As the localization of the nascent polypeptides is different between viral proteins with transmembrane domains (HA, NA and M2, which translocate to the ER and are transported through the Golgi to the plasma membrane) and soluble viral proteins (such as NP, the polymerase subunits, and NS1), mechanisms linking the translation of particular viral mRNA with subsequent protein localization rely on signal sequences recognized by the cell.