As for reinitiation at the displaced AUG codon, the data of Figure 6B (lanes 1-6) demonstrate that reinitiation certainly occurs at these downstream sites, because on this higher resolution gel the product size clearly decreases as the displacement is increased, although the efficiency is only 20-25% of the reinitiation frequency in the wild-type mRNA (lane 1). In fact, in this particular experiment there is much more downstream cistron product from the displaced AUG than product from the putative -1 AUA reinitiation site, which was only just visible on the autoradiogram (highlighted by the asterisk). In reviewing the results of many experiments of this type we conclude that the relative use of the two potential reinitiation sites shows quite a large degree of variability, which seems related to the use of different batches of reticulocyte lysate and so may possibly be due to small batch to batch differences in the endogenous ionic conditions. The variability seems to mainly affect the yield of putative -1 AUA reinitiation product, whereas the yield of product from the displaced AUG codon is relatively invariant at ,20% of the wild-type mRNA yield. The two extremes of the variability range are well illustrated by Figure 6B (very minimal use of the -1 AUA site) and Figure 6C , where there is actually more putative reinitiation at the -1 AUA than at the displaced AUG codon.