Ribosomal frameshifting may be controlled by mechanisms found in the mRNA sequence (cis-acting). This generally refers to a slippery sequence, a RNA secondary structure, or both. A −1 frameshift signal consists of both elements separated by a spacer region typically 5–9 nucleotides long. Frameshifting may also be induced by other molecules which interact with the ribosome or the mRNA (trans-acting).