The FMN riboswitch (also known as RFN element) is a highly conserved RNA element that is found frequently in the 5'-untranslated regions of prokaryotic mRNAs that encode for flavin mononucleotide (FMN) biosynthesis and transport proteins. This element is a metabolite-dependent riboswitch that directly binds FMN in the absence of proteins. In Bacillus subtilis, the riboswitch controls gene expression by causing premature transcription termination within the 5' untranslated region of the ribDEAHT operon and precluding access to the ribosome-binding site of ypaA mRNA.