Posttranscriptional regulation at mRNA level is defined not only by sequence, but also by the structure of the mRNA. Structures in the untranslated regions control stability, translation and localization of an mRNA. Evolutionary conservation can be used to identify such regulatory active structures in mRNAs. For example, constitutive decay element (CDE) stem-loops are recognized by Roquin proteins in an exclusively shape-specific manner. Strikingly, some CDEs can serve a dual function in gene repression depending on their folding status.