That AUG1 is essential for PB1 expression is obvious; the moderate Kozak consensus surrounding it has presumably evolved to allow expression of one or more of the downstream ORFs via leaky scanning. This sequence element appears be additionally selected for via the contribution these nucleotides make to the segment 2 specific packaging signal. However, in light of the theory that RNA viruses gain additional genes through selection of unused or poorly expressed ORFs (54) and that a selective advantage for PB1-F2 or N40 is not always obvious (7) (8) (9) 17, 18, 55) , it is not clear which functional element came first. AUG2 or sORF1 seems to be of no significance as a translational element since modulation of the AUG made no difference to protein expression in vitro or in virus infected cells. Similarly, removal of the stop codon had no effect on segment 2 protein expression, genome packaging or virus replication (data not shown). The AUG may be retained because PB1 function requires an aspartate residue at position 2 [this study; (21, 51, 52, 56) ] and because the wobble position of codon 2 has become fixed through its secondary role in the segment packaging signal. Retention of the stop codon is more difficult to explain, although positions 10 and 11 require leucine and lysine respectively (21, 51, 52, 56) and of the twelve possible permutations of this codon pair, only two do not result in a termination codon.