This signal is created via a chemical change, involving the addition of a molecule called 'ADP-ribose' to the DNA. The DNA is packaged up in a complex called 'chromatin' which contains proteins; the team found that the PARP3 enzyme adds the 'ADP-ribose' molecule to one of these proteins – 'histone H2B'.
By marking the precise site of damage the enzyme flags the problem up to specialised DNA repair enzymes that will move in to repair the damage, protecting the cell from potentially dangerous DNA breaks.