Recently, we showed that AUG5 of segment 2 is also used to initiate translation of a protein product called PB1-N40, made at $5% of the abundance of PB1 (7) . AUG5 is in frame with AUG1, and so N40 is a truncated form of PB1, lacking the first 39 amino acids of the longer polypeptide ( Figure 1A ). The 'missing' region is important for the interaction of PB1 with PA (19) , and therefore N40 should not be able to form the stable complex with PA necessary for efficient nuclear import and polymerase function (20, 21) . Indeed, N40 predominantly localized to the cytoplasm, and was not transcriptionally active (7) . A function for PB1-N40 has not yet been identified, although PB1-N40 null viruses retaining an intact PB1-F2 ORF displayed delayed single cycle growth kinetics (7) .