The IAV vRNA and cRNA segments form ribonucleoprotein (RNP) complexes by association to the polymerase and to multiple copies of the nucleoprotein (NP). These RNPs may be regarded as independent molecular machines responsible for transcription and replication of each segment. The viral RNA polymerase, which consists of the PA, PB1 and PB2 subunits, recognizes the RNA promoter, and stabilizes a supercoiled conformation of the RNPs. Different models have been proposed for the regulation of transcription and replication. One model suggests that the RNA polymerase switches from a transcriptase, used for mRNA synthesis, to a replicase, used for cRNA and vRNA synthesis, which is triggered by newly synthesized NP protein [13] . Another model suggests that cRNAs can be directly synthesized from incoming vRNAs, but require newly synthesized polymerase and NP to be stabilized in RNPs [14] . More recently, Jorba and colleagues proposed a model, in which a template RNP is replicated in trans by a soluble polymerase complex, whereas transcription of the vRNA occurs in cis and the resident polymerase complex is responsible for mRNA synthesis [15] .