Numerous in vitro studies have shown that human cell lines lacking functional ATM are extremely radiosensitive. In this issue, Moding et al. demonstrate using a murine model of sarcoma that deletion of the Atm gene has much less of a radiosensitizing effect on normal cardiac endothelia than on rapidly proliferating tumor endothelia. This work confounds our assumptions about the generality of the role of ATM in radiation sensitivity and the potential use of ATM inhibitors as radiosensitizers.