Our genomes are minefields, studded with potentially damaging DNA sequences over which hundreds of thousands of sentries stand guard. These sentries, called epigenetic marks, attach to the double helix at such spots and prevent the underlying DNA sequences from springing into destructive action. However, there are times when the sentries must leave their posts. So what defends us then? Professor Rob Martienssen and postdoctoral researcher Andrea Schorn have the answer.