CRISPR is the latest and most powerful technique for changing the genetic code of living things. This method of gene editing is already showing great promise in treating people with gene-based diseases, from sickle cell disease to cancer. However, in 2018 the use of CRISPR to edit the genes of two human embryos, which were subsequently born as two girls in China, caused outrage. The experiment was done in secrecy and created unintended changes to the children's genomes - changes that could be inherited by their children and their children's children. The scandal underlined the grave safety and ethical concerns around heritable genome editing, and called into doubt the ability of the scientific community to self-regulate this use of CRISPR. CRISPR gene editing might also be used to rapidly and permanently alter populations of organisms in the wild, and indeed perhaps whole ecosystems, through a technique called a gene drive. A gene drive is a way of biasing inheritance, of getting a gene (even a deleterious one) to rapidly multiply and copy itself generation after generation, sweeping exponentially through a population. In theory, this could be used to eradicate species such as agricultural pests or disease-transmitting mosquitoes, or to alter them in some way: for example, making mosquitoes unable to carry the malaria parasite. But do we know enough about the consequences of releasing a self-perpetuating genetic technology like this into the environment, even if gene drives could, for example, eradicate insects that spread a disease which claims hundreds of thousands of deaths every year? And who should decide whether gene drives should be released? Picture: DNA molecule, Credit: KTSDesign/SCIENCEPHOTOLIBRARY/Getty Images