The first major discrepancy between the infection and in vitro data concerned the role of AUG3 in controlling expression of the downstream ORFs. The virus lacking AUG3 (T72C) showed a statistically significant 3-fold increase in PB1-F2 accumulation relative to WT (compare lanes 11 and 14) , and this was consistent both with the in vitro data and the proposed role for leaky ribosomal scanning in accessing AUG4. Unlike in vitro however, there was a concomitant (and statistically significant) $2-fold reduction in N40 levels, despite normal PB1 accumulation. This result was not possible to reconcile with a model where leaky ribosomal scanning was the sole contributor to N40 expression, because removal of an upstream ORF should, at worst, leave N40 levels unchanged. Instead, we hypothesized that in vivo, ribosomes terminating at the end of sORF2 are able to reinitiate at AUG5, bypassing AUG4 because of the time taken to reacquire the necessary initiation factors (22) . In the absence of AUG3, more ribosomes initiate at AUG4 due to leaky ribosomal scanning, and so F2 levels increase. However, no ribosomes are available to reinitiate at AUG5, and so N40 levels decline. Supporting this hypothesis, when sORF2 was fused to PB1-F2 (A78T), N40 levels were reduced >2-fold ( Figure 4A, lane 16) . This virus would also be unable to express N40 by reinitiation from sORF2, due to the removal of the stop codon. However, reintroducing a stop codon using the G101T mutation reinstated N40 levels to 90% of wild-type (lane 17).