Viral RNP is assembled in the host cell nucleus through the interaction of full-length negative strand viral RNA (vRNA) and the viral nucleocapsid (NP) and matrix (M1) proteins. Studies of interactions of the purified components in vitro and of tissue culture model systems expressing various combinations of the components have established roles for both NP and M1 proteins in the assembly of a complex that has the physical properties of vRNP purified from virions and that can be exported from the host cell nucleus (Whittaker, 1996; Huang, 2001; Baudin, 2001). Viral RNP complexes have been found in the nucleoplasm, and also in the nuclear periphery, associated with the nuclear matrix or chromatin, particularly for vRNA-containing complexes and M1 protein (Elton, 2005; Garcia-Robles, 2005; Takizawa et al., 2006).