Based on the length of stem 1, the RNA pseudoknots capable of stimulating À1 PRF can be classified into two groups. One group of pseudoknot requires a long stem 1 to stimulate À1 PRF efficiently and can be represented by the infectious bronchitis virus pseudoknot (IBV-PK), which contains 11 bp in stem 1. Deviation from the optimal 11-12 bp in stem 1 will lead to the impairment of IBV-PK frameshift efficiency (24) . However, no high-resolution structural information is available for this class of pseudoknot. In contrast, the other group of pseudoknots, which includes mouse mammary tumor virus pseudoknot (MMTV-PK), beet western yellow virus (BWYV-PK) and simian retrovirus pseudoknot (SRV-PK), has a short stem 1 of <7 bp in length (16) . The analysis of high-resolution structures of several short stem 1 possessing pseudoknots indicated that they all share a common structural feature, involving tertiary loop-helix interactions (between stem 1 and loop 2) close to the helical junction (22, (25) (26) (27) (28) . The requirement of these tertiary interactions in À1 PRF activity has been verified by mutagenesis analysis (29) (30) (31) (32) . However, the relative small size of this group of pseudoknots makes it difficult to rationalize the results of extensive mutagenesis analysis because a mutant may not necessarily still form a pseudoknot. Furthermore, NMR structures of a luteoviral pseudoknot and its poorly functional variant suggest that both pseudoknots possess essentially identical global structure (27, 28) . Therefore, how structurally similar pseudoknots produce distinct À1 PRF efficiency remains an open question.