Repairing faulty genes to prevent and cure disease is something researchers have been working towards for many years. While Class 2 CRISPR systems show great promise as gene editing tools in human cells, a research team has now demonstrated that a Cas3-based Class 1 CRISPR system may provide a more efficient and safer alternative, carrying out successful repair of a gene mutation responsible for Duchenne muscular dystrophy in patient-derived cells.