The HIV-1 frameshift event requires two cis-acting elements in the viral mRNA: a slippery sequence, UUUUUUA, where the frameshift occurs (1, reviewed in 2,3), followed by an irregular stem-loop (9) (10) (11) , the frameshift stimulatory signal, that makes the ribosomes pause over the slippery sequence and controls the frameshift efficiency. Only a fraction of the ribosomes that encounter the stimulatory signal make a frameshift. After the pause, the ribosomes unfold the signal, which can reform after their passage.