It has been suggested that leaky ribosomal scanning is responsible for PB1-F2 and PB1 N40 expression (6, 7, 17) . In the scanning model of translation initiation, ribosomes bind to the 5 0 end of mRNA and move along until they recognize a start codon (22) . The sequence context of the AUG affects the probability that it will be recognized as a bona fide initiation codon; the Kozak consensus GCC(A/G)CCAUGG, is thought to be optimal, with a purine at À3 and G at +4 exerting the strongest effects (23, 24) . In support of the ribosomal scanning hypothesis, AUG1 is set in a medium strength Kozak consensus, lacking a purine at À3 ( Figure 1A and B), while mutation of AUG4 has been shown to lead to upregulation of N40 translation from AUG5 (7) . However, the presence of two short ORFs (sORFs) initiated by AUGs 2 and 3 upstream of the PB1-F2 and N40 AUGs ( Figure 1A and B) is suggestive of additional regulatory Kozak consensus sequence (green, strong consensus, with A/G at À3 and G at +4; yellow, medium consensus with either A/G at À3 or G at +4; red is a weak consensus U at -3 and +4). Adapted from (7) . (B) Nucleotide sequence and site of mutations used in this study. The 5 0 -end of segment 2 mRNA is shown in positive sense and as cDNA, since all mutations were introduced into a plasmid clone of the segment. (C) Summary of the predicted effect of the mutations used in this study on AUG strength and ORF structure (non synonymous changes in PB1 are indicated after red asterisks).