Here, we demonstrate that an RNA pseudoknot, hTPK-DU177 (DU177), which is derived from human telomerase RNA (33, 34) , can function as an efficient À1 PRF stimulator. Because well-characterized À1 PRF pseudoknots containing short stem 1 all possess relative short loop 1 (22, (25) (26) (27) (28) , information for the contribution of loop 1-stem 2 interactions to À1 PRF activity of a pseudoknot is limited (18) . Therefore, the long loop 1 of DU177 and its extensive stem-loop interactions with the stem 2 make this well-defined pseudoknot an excellent model to explore this issue. Using DU177 and its variants, we show that the À1 PRF efficiency of DU177 can be impaired by the disruption of specific base-triple interactions flanking the helical junction region. In contrast, a DU177 mutant, lacking most of the triple interactions identified in DU177, has essentially no frameshift activity but still adopts a pseudoknot conformation. Furthermore, the triplex in DU177 is then mimicked and constructed intermolecularly (35) , and is used to decouple triplex formation from pseudoknot topology to resolve their roles in À1 PRF activity. Therefore, our work provides a platform to characterize the contributions of two structural determinants of a À1 PRF pseudoknot. Finally, the results from this work also imply that a non-coding RNA may act in-trans to stimulate À1 PRF on targeted mRNAs by specific intermolecular triplex formation.