The origins of the RNA silencing pathway are in defense against invading viruses and in response, viruses have evolved counter-measures to interfere with the host pathway. The p19 protein is expressed by tombusviruses as a suppressor of RNA silencing and functions to sequester small RNA duplexes, thereby preventing induction of the pathway. p19 exhibits size-specific and sequence-independent binding of its small RNA ligands, binding with high affinity to duplexes 20-22 nucleotides long. p19's binding specificity and its ability to sequester small RNAs has made it a unique protein-based tool for probing the molecular mechanisms of the highly complex RNA silencing pathway in a variety of systems. Furthermore, protein engineering of this 'molecular caliper' promises novel applications in biotechnology and medicine where small RNA molecules are of remarkable interest given their potent gene regulatory abilities.