C24. This nucleotide is extruded from loop 2 by the stacking interaction of the continuous run of adenines and is not involved in any interaction. Thus, from a structural point of view, the presence of C24 in loop L2 seems superfluous or even a nuisance by hindering the stacking of the A21-A26 adenosines. Changing C24 to U (SF111) did not affect frameshifting ( Figure 1C) , suggesting that also a uridine at this position is flipped out. Replacing C24 by G (SF112) or deleting it (SF202) had no significant effect on frameshifting while an adenosine at this position slightly enhanced frameshifting activity (SF113), possibly as a result of extended adenosine stacking. These results show, as expected, that C24 is irrelevant and dispensable for frameshifting.