GENE drives are, at heart, a particularly selfish sort of gene. Most animals have two copies of most of their genes, one on the set of chromosomes they got from their mother, one on those from their father. But they put only one version of each gene—either the maternal one or the paternal one, at random—into each of their own gametes (sperm or eggs). Some genes, though, seek to subvert this randomising in order to get into more than 50% of the gametes, and thus more than 50% of the next generation.

In 1960 George Craig, an American entomologist, suggested that such subversive genes might be a way of controlling the populations of disease-carrying mosquitoes, for example by making them more likely to have male offspring than female ones. In 2003 Austin Burt, at Imperial College, described how a gene drive that could cut a place for itself in a chromosome and copy itself into the resulting gap could, in the right circumstances, drive a species to extinction.

A fascinating idea, but one hard to put into practice—until, in 2012, a powerful new gene-editing tool called CRISPR-Cas9 became available. Gene drives based on CRISPR-Cas9 could easily be engineered to target specific bits of the chromosome and insert themselves seamlessly into the gap, thus ensuring that every gamete gets a copy (see diagram). By 2016, gene drives had been created in yeast, fruitflies and two species of mosquito. In work published in the journal Nature Biotechnology in September, Andrea Crisanti, Mr Burt and colleagues at Imperial showed that one of their gene drives could drive a small, caged population of the mosquito Anopheles gambiae to extinction—the first time a gene drive had shown itself capable of doing this. The next step is to try this in a larger caged population.

Read more in our briefing, Extinction on demand