Riboswitches are segments of the 5'-untranslated region of certain bacterial mRNAs that upon recognition of specific ligands modify the expression of a protein(s) encoded in the message. These proteins are responsible for the biosynthesis or transport of ligands, which are typically organic molecules but could also be metal ions. Riboswitch-mediated control of gene expression might be thermodynamic or kinetic, depending on the rate of transcription elongation by RNA polymerase and the structures adopted by the riboswitch RNA. Certain 5'-untranslated regions harbor two riboswitches in tandem that bind to different ligands. Thus, RNA sensors can respond to metabolic changes by modifying gene expression in ways previously thought to be exclusive of proteins.