Telomerase is activated in many human cancers, and the possibility of targeting tumours with telomerase inhibitors is attractive. But a study in September's Nature Genetics shows that Terc−/− mice, which lack functional telomerase, are especially sensitive to ionizing radiation. The scientific implication is that functionally intact telomeres and the response to ionizing radiation are somehow linked; the clinical one is that it may not be wise to treat cancer patients with both telomerase inhibitors and ionizing radiation.