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The tumor suppressor p53 is required for the maintenance of genomic integrity following DNA damage. One mechanism by which p53 functions is to induce a block in the transition between the G₁ and S phase of the cell cycle. Previous studies indicate that the Krüppel-like factor 4 (KLF4) gene is activated following DNA damage and that such activation depends on p53. In addition, enforced expression ofKLF4 causes G₁/S arrest. The present study examines the requirement of KLF4 in mediating the p53-dependent cell cycle arrest process in response to DNA damage. We show that the G₁ population of a colon cancer cell line, HCT116, that is null for the p53 alleles (−/−) was abolished following γ irradiation compared with cells with wild-type p53 (+/+). Conditional expression ofKLF4 in irradiated HCT116 p53−/− cells restored the G₁ cell population to a level similar to that seen in irradiated HCT116 p53+/+ cells …