Gene editing has begun to be tested in clinical trials, using CRISPR-Cas9 and other technologies to directly edit DNA inside people’s cells, and multiple trials are recruiting or in planning. A new study raises a note of caution, finding person-to-person genetic differences that may undercut the efficacy of gene editing or, more rarely, cause potentially dangerous ‘off target’ effects. It suggests that gene editing may need to be adapted to each patient’s genome.