Champer, J. et al. PloS Genet. http://dx.doi.org/10.1371/journal.pgen.1006796 (2017).

Super-Mendelian inheritance, during which one allele of a heterozygous locus is passed down to the majority of offspring, has a number of interesting applications such as controlling the fertility of disease vectors. Such gene drives are relatively easy to design with the CRISPR system that cuts the undesirable allele followed by homology-driven repair to replace it with the allele of choice. However, resistance alleles, no longer cut by Cas9, also develop rapidly. Champer et al. now quantify the rate of resistance-allele formation in flies using two gene drives to replace the X-linked yellow gene with a fluorescent protein. From the high percentage of resistance alleles in germline cells and embryos, the researchers conclude that new designs are needed if a gene drive is to spread in a wild population.