CRISPR and Caspase meet Many prokaryotes use CRISPR RNA–bound proteins to sense viral RNA instead of DNA to set an immune response in motion that protects from virus infection. Although these ribonucleoproteins are typically composed of many protein subunits, van Beljouw et al . discovered that CRISPR-Cas type III-E systems are formed by a large, single-component effector protein capable of double RNA cleavage. Distinct from other systems, this effector forms a complex with a peptidase from the caspase family, raising the intriguing possibility that viral RNA activates a protease activity to prevent virus propagation by host suicide. —DJ