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 the planning stages. A new study led by Boston Children’s Hospital and the University of Montreal raises a note of caution, finding that person-to-person genetic differences may undercut the efficacy of the gene editing process or, in more rare cases, cause a potentially dangerous “off target” effect.