The Argonaute endonuclease of the archaebacterium Natronobacterium gregoryi (NgAgo) is a precise and efficient tool for genome editing in mammalian cells. Using plasmid cleavage assays, Gao et al. found that — in contrast to the guide RNA (gRNA) needed by the Cas9 nuclease — NgAgo can cleave targets using single-stranded DNA of ~24 nucleotides in length as a guide. NgAgo was found to create site-specific DNA double-strand breaks in various mammalian cell lines with similar efficiency to the Cas9–gRNA system. Nucleotide mismatch at any position between the guide DNA (gDNA) and the target sequence substantially reduced target cleavage efficacy, and three consecutive mismatches abolished nuclease activity completely, suggesting high fidelity of the NgAgo–gDNA system. Correct insertion of donor DNA by homology-directed repair into a targeted genome locus was also achievable.