Elsewhere, we reported that multiple serial in vivo passage of a squamous cell carcinoma cells (SCC61) concurrent with ionizing radiation (IR) treatment resulted in the selection of radioresistant tumor (nu61) that overexpresses the signal transducer and activator of transcription 1 (Stat1)/IFN-dependent pathway. Here, we report that (a) the Stat1 pathway is induced by IR, (b) constitutive overexpression of Stat1 is linked with failure to transmit a cytotoxic signal by radiation or IFNs, (c) selection of parental cell line SCC61 against IFN-alpha and IFN-gamma leads to the same IR- and IFN-resistant phenotype as was obtained by IR selection, and (d) suppression of Stat1 by short hairpin RNA renders the IR-resistant nu61 cells radiosensitive to IR. We propose a model that transient induction of Stat1 by IFN, IR, or other stress signals activates cytotoxic genes and cytotoxic response. Constitutive overexpression of Stat1 on the other hand leads to the suppression of the cytotoxic response and induces prosurvival genes that, at high levels of Stat1, render the cells resistant to IR or other inducers of cell death.