Before revision: One of the key factors in -1 programmed ribosomal frameshifting (PRF) is a pseudoknot (PK) RNA . A number of single molecule  experiments have been performed on PKs to decipher the mechanism of PRF.
After revision: One of the key factors in -1 programmed ribosomal frameshifting (PRF) is a pseudoknot (PK) RNA to force, a number of single molecule experiments have been performed on PKs to decipher the mechanism of PRF.