The groundbreaking clinical trial, described on 31 October in the American Journal of Human Genetics, will deploy an offshoot of the CRISPR-Cas9 gene-editing technique called base editing, which allows scientists to make precise, single-letter changes to DNA sequences. The study is expected to begin next year, after its organizers spent months negotiating with US regulators over ways to simplify the convoluted path a gene-editing therapy normally has to take before it can enter trials.