Influenza A virus contains eight single stranded, negative sense RNAs. Each RNA is associated with viral RNA-dependent RNA polymerase which consists of PA, PB1 and PB2 proteins. PB1 is the subunit involved in the catalytic activity of nucleotide polymerization and is involved in the initiation of transcription. Both PB1 and PB2 can be crosslinked to synthetic RNA with the 3' terminal sequence of viral RNA. PB1 and PB2 may also be involved in recognition of the promoter and/or replication origin on template viral RNA. PB2 is the cap 1-recognition protein, which binds to the cap structure of host mRNA. The cap structure then acts as a primer for transcription to produce viral mRNA.