NORMALLY growing cells promptly cease DNA synthesis when exposed to genotoxic stresses, such as radiation, and this cell-cycle arrest prevents the accumulation of mutations(1,2). The transcription factor interferon regulatory factor (IRF)-1 is essential for the regulation of the interferon system(3-5), inhibits cell growth, and manifests tumour-suppressor activities(6,7). Here we show that mouse embryonic fibroblasts (EFs) lacking LRF-1 are deficient in their ability to undergo DNA-damage-induced cell-cycle arrest. A similar phenotype has been observed in EFs lacking the tumour suppressor p53 (refs 8, 9), although the expression of IRF-1 and p53 are independent of one another. Furthermore, we show that transcriptional induction of the gene encoding p21 (WAF1, CIP1)(10-12), a cell-cycle inhibitor, by gamma-irradiation is dependent on both p53 and IRF-1, and that the p21 promoter is activated, either directly or indirectly, by both in a transient cotransfection assay. These two tumour-suppressor transcription factors therefore converge functionally to regulate the cell cycle through the activation of a common target genes.