BWYV (29) , UAAAAACAC in ScYLV (25) , ACUCAAA A in MMTV (32) and AAACAA in HIV-1 type O (33) . Interestingly, L2 of the human telomerase pseudoknot, not involved in frameshifting, consists of CAAACAAA. This pseudoknot has recently been shown to function in frameshifting in vitro (22) . Also here the adenosines formed triple interactions with base pairs in stem S1 and were essential for frameshifting (22, 34) . The effect of substituting the second C in L2 has not been investigated; its presence may be required to prevent slippage of the RNA polymerase.