The authors identified a protein in the archaeon Haloquadratum walsbyi that cleaves mRNAs at a specific 7-base sequence. This protein, MazF-hw, is a homologue of mRNA interferases identified in bacteria, but the previously identified proteins recognize shorter sequences. The longer sequence may achieve a similar level of mRNA regulation specificity to that conferred by microRNAs or by small interfering RNAs. Indeed, the MazF-hw target site is enriched in particular genes, including in a rhodopsin transcription activator. It is possible that highly specific mRNA interferases could be developed as tools to control gene expression.