When mammalian cells are under stress they activate the tumour-suppressor gene p53. But how does the cell signal to p53 that it is under attack? A new study implicates the DNA-dependent protein kinase (DNA-PK), which is activated by DNA damage and can bind p53. Damaged cells that don't contain DNA-PK accumulate an inactive form of p53, indicating that DNA-PK is crucial for the activation of p53.