Orthomyxoviridae RNA polymerase with the subunit composition of PB1-PB2-PA is a unique multifunctional enzyme with the activities of both synthesis and cleavage of RNA, and is involved in both transcription and replication of the RNA genome. Transcription is initiated by using capped RNA fragments, which are generated after cleavage of host cell mRNA by the RNA polymerase-associated capped RNA endonuclease [<cite idref="PUB00005639"/>]. It would appear that two separate sequences, one N-(242-282) and the other C-terminal (538-577) proximal segments of PB2 subunit, constitute the RNA cap-binding site of the Influenza virus RNA polymerase [<cite idref="PUB00006567"/>].