The P protein of Paramyxovirinae plays multiple roles in both transcription and replication: It is an essential subunit of the viral polymerase complex and acts as a bridge between the nucleocapsid template (N NUC ) and the polymerase complex L-P. P is a modular protein, consisting of an intrinsically unstructured N-terminal moiety (PNT) (Karlin et al. 2002b , and of a well conserved C-terminal moiety (PCT) which contains all the regions required for transcription (Curran 1996) . Paramyxovirinae PCT have a modular organization, consisting of alternating disordered and structured regions . In particular, they possess a coiled-coil domain (referred to as PMD, for P multimerization domain) responsible for both oligomerization and binding to L (Smallwood et al. 1994; Liston et al. 1995) , and a C-terminal globular region (referred to as XD), that is involved in binding to both monomeric and assembled forms of N (Curran et al. 1994 (Curran et al. , 1995a Harty and Palese 1995; Tuckis et al. 2002; Johansson et al. 2003; Kingston et al. 2004b) . Figure 1 shows a schematic representation of the MV N NUC -P complex, which highlights the role of N TAIL in the recruitment of P via the binding to XD.