Illustration of a riboswitch with its ligand (red) bound. Riboswitches are segments of bacterial mRNA (messenger ribonucleic acid) that regulate the transcription of the mRNA's protein product. When the aptamer domain (blue) of the riboswitch binds to its ligand it causes structural changes in the expression platform (orange) of the riboswitch. These changes affect the coding region of the mRNA molecule, either inhibiting or enhancing expression of the protein product.