The p53-mediated pathway cell cycle arrest and apoptosis is central to cancer and an important point of focus for therapeutics development. The p14ARF ("ARF") tumor suppressor induces the p53 pathway in response to oncogene activation or DNA damage. However, ARF is predominantly nucleolar in localization and engages in several interactions with nucleolar proteins, whereas p53 is nucleoplasmic. This raises the question as to how ARF initiates its involvement in the p53 pathway. We have found that UV irradiation of cells disrupts the interaction of ARF with two of its nucleolar binding partners, B23 (NPM, nucleophosmin, NO38, numatrin) and topoisomerase I, and promotes an immediate and transient subnuclear redistribution of ARF to the nucleoplasm, where it can engage the p53 pathway (Lee et al, Cancer Res 65:9834-42; 2005). The results support a model in which the nucleolus serves as a p53 upstream sensor of cellular stress, and add to a growing body of evidence that nucleolar sequestration of ARF prevents activation of p53. The results also have therapeutic implications for therapies based on exploiting p53 and other cellular stress response pathways to suppress cancer.