With the advent of clustered, regularly interspaced, short palindromic repeats (CRISPR)–CRISPR-associated protein 9 (Cas9) technology, researchers can construct gene drives that can bias the inheritance of edited alleles to alter entire populations. As demonstrated with the mutagenic chain reaction in Drosophila4, the CRISPR-Cas9 system can propagate genomic modification together with the genome-editing machinery itself. Although gene drives might have the potential to control insect-borne diseases and agricultural pests, substantial concerns have been raised over unanticipated ecological consequences as a result of drive use. Here we report the development of a potential Cas9-based gene drive 'brake' that remains inert in a wild-type genome but is activated by Cas9 to both cleave the genomic cas9 sequence and to convert an incoming cas9 allele into a brake. This means that the propagation of the brake is favored in a cas9-carrying population.