Reverse genetic studies and VLP assembly studies suggest that M protein promotes assembly by interacting with viral ribonucleoprotein (RNP) and S glycoproteins at the budding site (de Haan et al., 1999; Escors et al., 2001a; Escors et al., 2001b; Kuo and Masters, 2002; Narayanan et al., 2000; Nguyen and Hogue, 1997; Opstelten et al., 1995; Sturman et al., 1980) , and by forming a network of M-M interactions that is capable of excluding some host membrane proteins from the viral envelope (de Haan et al., 2000; Neuman et al., 2008b) .