The discovery of CRISPR-Cas9 has made gene editing very easy. Unfortunately, the molecular tool has recently been found to be less precise than previously assumed. It can lead to unwanted mutations in a cell’s DNA. Researchers at Delft University of Technology have now identified a mechanism that causes such mutations when CRISPR-Cas9 is used incorrectly. This can cause dormant genes to become expressed, which is potentially very dangerous. The researchers have created a checklist based on their findings. Using this checklist will prevent the harmful mechanism from being activated and makes gene editing using CRISPR-Cas9 safer.