. This may represent a nonmutational mechanism for abrogating p53 tumor suppressor function. To test this hypothesis, we established the first available in vitro model that accurately reflects the wild-type p53 sequestration found in NB tumors. We characterized a series of human NB cell lines that overexpress wild-type p53 and show that p53 is preferentially localized to discrete cytoplasmic structures, with no detectable nuclear p53. These cell lines, when challenged with a variety of DNA strand-breaking agents, all exhibit impaired p53-mediated G 1 arrest. Induction analysis of p53 and p53-responsive genes show that this impairment is due to suppression of nuclear p53 accumulation. Thus, this naturally occurring translocation defect compromises the suppressor function of p53 and likely plays a role in the tumorigenesis of these tumors previously thought to be unaffected by p53 alterations.Wild-type p53 plays a key suppressor role in cell growth and tumor formation. p53 acts as a cell cycle checkpoint after DNA damage, induces G 1 arrest or apoptosis (reviewed in reference 63), and is required to maintain genomic stability (34, 70). The mechanism underlying its central growth suppression is largely based on p53's function as a potent transcriptional regulator (9, 13) of crucial growth-inhibitory genes such as Waf-1, a universal inhibitor of cyclin kinase complexes (10,19,47,67). The domain structure of p53 reveals an N-terminal transactivation region, a central sequence-specific DNA-binding region, and a C-terminal oligomerization region that also harbors several nuclear localization domains (reviewed in reference 50). Accordingly, p53 is a nuclear phosphoprotein, and nuclear localization is essential for its growth-suppressing activity in late G 1 (14,15,(53)(54)(55)(56). Site-directed mutagenesis of the primary nuclear localization signal (at residues 316 to 322) caused cytoplasmic retention and completely destroyed the transformation-suppressing activity of wild-type p53 (56). In normal cells, p53 levels are tightly regulated as a result of a short half-life of 15 to 30 min (49) and are not detectable by immunocytochemistry (31).Disruption of the p53 response pathway strongly correlates with tumorigenesis. Indeed, functional inactivation of p53 is the single most common event in human malignancies and occurs in at least 50% of all cancers (20). Mutational inactivation is the most common mechanism and occurs in a large spectrum of sporadic and familial cancers of, e.g., the breast, gastrointestinal tract, lung, brain, and soft tissues (22). Deletion of one allele accompanied by a missense mutation in the central DNA-binding domain of the remaining allele is classical. Most point mutations prolong the half-life of p53, leading to nuclear accumulation which now becomes readily detectable by immunocytochemistry. Though less frequent, mutation-independent mechanisms are also utilized in human malignancies. Here, the common theme is sequestration of wild-type p53 protein by another protein which abrogates its supp...