Radiotherapy works by damaging the DNA of cancer cells. It’s an effective strategy overall, but many cancers have subsets of cells that are able to survive initial radiotherapeutic regimens by using DNA damage repair mechanisms. This often results in resistance to further radiation as cancerous growth recurs. A team led by Roel Verhaak analyzed patient cancers before and after radiotherapy and found a deletion signature in many post-treatment samples. The finding indicates that combining radiotherapy with DNA repair inhibition may improve efficacy.