In the host cell nucleus, the viral negative-strand RNA (vRNA) serves as a template for the synthesis both of capped, polyadenylated viral messenger RNA and of full-length positive-strand RNA or complementary RNA (cRNA). The cRNA is associated with the same viral proteins as the vRNA. It serves as a template for the synthesis of new vRNA molecules, which in turn serve as a template for mRNA particularly early in infection, and cRNA. Viral RNA polymerase subunits (PB1, PB2, and PA) and nucleoprotein (NP) enter the host cell nucleus and catalyze all three of these reactions. During initial infection, these proteins enter the nucleus as part of the viral RNP complex. After the first round of viral mRNA synthesis (primary transcription) and translation, newly synthesized viral polymerase proteins and NP localize to the nucleus to catalyze further mRNA transcription and vRNA/cRNA replication. Late in the infection process, the synthesis of vRNA and nuclear export of newly synthesized vRNP (vRNA complexed with NP and viral polymerase) is increased relative to transcription (Krug, 1981; Braam, 1983; Kawakami, 1983; Huang, 1990; Cros, 2003; Fodor, 2004; Deng, 2005; Amorim, 2006; reviewed in Neumann, 2004; Engelhardt, 2006; Buolo, 2006).