Small RNAs (sRNAs) are used by plants, nematodes and arthropods in cellular defence against viruses, but in chordates, sRNAs were replaced with a protein-based system to inhibit viral replication. Here, Benjamin tenOever describes how the lack of interplay between cellular sRNAs and RNA viruses permits the engineering of vectors that can deliver their own sRNAs or are controlled by the sRNAs present in the host.