Influenza virus-associated RNA polymerase is composed of one molecule each of three viral P proteins and carries the complete activity of capped RNA-primed vRNA-directed transcription. The RNA polymerase holoenzyme also carries capped RNA endonuclease to generate capped oligonucleotide primers for transcription and 3'-to-5' exonuclease to remove erroneously polymerized nucleotides at nascent RNA 3' termini prior to the addition of correct substrates. PB1 is the core subunit for not only RNA synthesis but also the assembly of PB2 and PA into the holoenzyme complex, while PB2 plays a key role in capped RNA cleavage. The transcriptase is converted into the RNA replicase with the full activity of replication, ie vRNA-directed cRNA synthesis and cRNA-directed vRNA synthesis, after interaction with an as yet unidentified host factor(s).