Gefitinib inhibited IR-induced ATM phosphorylation in the two NSCLC cell lines (NCI-H460 and VMRC-LCD) that were radiosensitized by this agent, but IR-induced ATM phosphorylation was intact after gefitinib pretreatment in an NSCLC cell line (A549) that was not radiosensitized by this drug. The gene discussed is ATM; the disease is non-small cell lung carcinoma.