Plants have special corrective molecules at their disposal that can make retrospective modifications to copies of genes. However, it would appear that these “Tipp-Ex proteins” do not have permission to work in all areas of the cell, only being used in chloroplasts and mitochondria. A study by the University of Bonn has now explained why this is the case. It suggests that the correction mechanism would otherwise modify copies that have nothing wrong with them, with fatal consequences for the cell. The findings have now been published in “The Plant Journal.”