G1-C19. A previous sequence comparison showed that most frameshifter pseudoknots start with a 5 0 G-C3 0 base pair (30). Changing this base pair, which is not involved in any loop interaction, to C-G (SF209) affected frameshifting efficiency somewhat (73%). As stacking energies of this base pair are also lower (À2.4 versus À3.3 kcal/mol) this confers to the general notion that helix stability is an important determinant in frameshifting. Changing G1-C19 into an A-U base pair (SF211) or a G-G mismatch (SF107) reduced the efficiency even further to about 40%, again emphasizing the role of S1 stability.