The nucleoprotein (NP) of Influenza virus encapsulates the negative strand of the viral RNA and is essential for replicative transcription. It may also be involved in other essential functions throughout the virus life cycle. As well as binding ssRNA, NP is able to self associate to form large oligomeric complexes. NP is able to interact with a variety of other macromolecules of both viral and cellular origins. It binds the PB1 and PB2 subunits of the polymerase and the matrix protein M1. "NP has also been shown to interact with at least four cellular polypeptide families: nuclear import receptors of the importin class, filamentous (F) actin, the nuclear export receptor CRM1 and a DEAD box helicase BAT1/UAP56" (Portela et al 2002).