Riboswitches are structured RNA elements usually located in the untranslated regions (UTR) of mRNAs that act as direct sensors of metabolites to control gene expression. More than a dozen distinct classes have been found to regulate the expression of genes involved in key metabolic processes. In most instances each riboswitch senses one metabolite and functions as a simple genetic switch. However, we have recently identified several examples of mRNAs that incorporate riboswitches or their components in tandem. The 5'-UTR of the Bacillus clausii metE mRNA contains a riboswitch for S-adenosylmethionine (SAM) followed by another for adenosylcobalamin (coenzyme B₁₂). This tandem arrangement allows binding of either SAM or coenzyme B₁₂ to repress metE expression. The resulting genetic readout of different chemical inputs corresponds to a Boolean NOR logic gate. These findings and related discoveries of other tandem motifs reveal how RNA can make sophisticated genetic decisions without involvement of protein factors.