Cas proteins like CRISPR-Cas9 have great potential for gene therapy to treat human disease and for altering crop genes, but the gene-targeting and gene-cutting Cas proteins are often large and hard to ferry into cells with viral vectors such as adenovirus. Scientists have now discovered a hypercompact Cas protein, Cas-phi, that should work better. It is half the size of Cas9 and apparently evolved inside a bacteriophage, yet efficiently snips double-stranded DNA.