<p> Paramyxovirinae has a negative-sense ssRNA genome that is packaged by the viral nucleoprotein (N) within a helical nucleocapsid. The N-RNA (nucleoprotein-RNA) complex is used as a template for both transcription and replication. During viral genome replication, the synthesis of viral RNA and its encapsidation by N are concomitant. Viral transcription and replication are carried out by viral RNA-dependent RNA polymerase, which consists of two proteins: L polymerase and phosphoprotein P. The L polymerase carries the enzyme activity. Phosphoprotein P binds the viral nucleocapsid, and positions the L polymerase on the template for transcription and replication formed by nucleoprotein-RNA (N-RNA). </p><p>Phosphoprotein P, an indispensable subunit of the viral polymerase complex, is a modular protein organised into two moieties that are both functionally and structurally distinct: a well-conserved C-terminal moiety that contains all the regions required for transcription, and a poorly conserved, intrinsically unstructured N-terminal moiety that provides several additional functions required for replication. The N-terminal moiety is responsible for binding to newly synthesized free N(0) (nucleoprotein that has not yet bound RNA), in order to prevent the binding of N(0) to cellular RNA. The C-terminal moiety consists of an oligomerisation domain, an N-RNA (nucleoprotein-RNA)-binding domain and an L polymerase-binding domain. </p><p>This entry represents the XD domain found at the extreme C-terminal of the C-terminal moiety of phosphoprotein P from several Paramyxovirinae, including <taxon tax_id="11234">Measles virus</taxon> [<cite idref="PUB00020838"/>, <cite idref="PUB00031362"/>], <taxon tax_id="11232">Canine distemper virus</taxon> and <taxon tax_id="11191">Sendai virus</taxon> [<cite idref="PUB00030518"/>, <cite idref="PUB00042566"/>, <cite idref="PUB00003548"/>].</p> RNA polymerase, phosphoprotein P, C-terminal XD, paramyxovirinae