The most likely scenario suggested by our observations is that replication/transcription of one reporter segment interferes with that of another by sequestering UTR-binding proteins, probably polymerases, required for RNA synthesis. Several observations by us and others support this hypothesis: 1) increasing the amount of polymerase and NP proteins, but not of NP protein alone, alleviated the competition between different segments, 2) the polymerase proteins have been shown to bind to 59 and 39 UTRs of vRNAs, with most strong binding observed to the 59 UTR [33] , 3) introduction of mutations in the 39 UTR (NPph) that stabilize the panhandle structure and are predicted to result in increased polymerase binding [23] result in increased ability of the reporter segment to be replicated/transcribed in the presence of competitor segments ( [23] and this study), 4) introduction of similar mutations in the 59 UTR (NPphR) that are likely to interfere with polymerase binding [33] , had a negative effect on the competitive ability of the reporter construct, and 5) panhandle-stabilizing mutations in the 39 UTR (NPph), that increased the competitive ability of the reporter construct, partly compensated for replication-debilitating mutations in PB2 (R142A or E361A) [34, 35] , but not in NP (M331K or F488G) [36] (Fig. S9) , suggesting a link between the interaction of the UTR with polymerase and the ability to compete with other segments. Although our experimental system (i.e. the transfection assay) does not approach the complexity of the IAV infected cells with respect to number of vRNA segments and viral proteins present, our data suggest that IAV RNA segments compete with each other for available polymerases. This competition is expected to be most apparent early during infection, when only low amounts of polymerase are present. It is conceivable that at this stage of the infection the low level of RNA polymerase plays a critical role in the regulation of segment-specific replication and/or transcription. At later times during infection competition between vRNA segments is expected to be alleviated by the increased levels of the polymerase subunits, thereby ensuring the efficient replication/transcription of all genome segments. 