Project Summary: CRISPR/Cas9 technology has revolutionized genome editing and holds great promise for the develop- ment of new genetic therapies. It is now possible to make precise lesions at defined locations in many model organisms and cell lines. Physical and regulatory connections identified through numerous “omics” studies can now be tested directly in vivo, establishing their relative importance to cell and animal physiology. The nema- tode Caenorhabditis elegans is an exemplary tool for gene function research. The worm is easy to culture, has a short generation time, and a large brood size. RNAi is highly efficient in this species— pathways exist to take up double stranded RNA (dsRNA) from the environment and spread it throughout the worm, enabling RNAi by soaking the worms in dsRNA, or by feeding the worms bacteria that express dsRNA, a property that has ena- bled routine genome-wide RNAi screening. Now, thanks to CRISPR / Cas9, it is possible to make both targeted gene deletions and targeted gene replacements in C. elegans a relatively straightforward way. While this approach is now widely used in many C. elegans research labs throughout the world, there remains a major limitation. The need to microinject guides creates a bottleneck that prevents high throughput mutagenesis studies. Microinjection requires both specialized equipment and training. We propose to combine the most useful features of RNAi, including deliv- ery by soaking or feeding, with CRISPR/Cas9 mediated genome editing, to eliminate this research bottleneck. Our approach—termed siCRISPR—will enable genome-wide targeted mutagenesis studies that will enable functional testing of hypotheses generated through “omics” level sequencing methods.