The p53 protein integrates multiple upstream signals and functions as a tumor suppressor by activating distinct downstream genes^,,. At the cellular level, p53 induces apoptosis, cell cycle arrest and senescence. A rare mutant form of p53 with the amino acid substitution R175P, found in human tumors, is completely defective in initiating apoptosis but still induces cell cycle arrest^,. To decipher the functional importance of these pathways in spontaneous tumorigenesis, we used homologous recombination to generate mice with mutant p53-R172P (the mouse equivalent of R175P in humans). Mice inheriting two copies of this mutation (Trp53^515C/515C) escape the early onset of thymic lymphomas that characterize Trp53-null mice. At 7 months of age, 90% of Trp53-null mice had died, but 85% of Trp53^515C/515C mice were alive and tumor-free, indicating that p53-dependent apoptosis was not required for suppression of early onset of spontaneous tumors. The lymphomas and sarcomas that eventually developed in Trp53^515C/515C mice retained a diploid chromosome number, in sharp contrast to aneuploidy observed in tumors and cells from Trp53-null mice. The ability of mutant p53-R172P to induce a partial cell cycle arrest and retain chromosome stability are crucial for suppression of early onset tumorigenesis.