Abstract ATM and p53 are critical regulators of the cellular DNA damage response and function as potent tumor suppressors. In cells undergoing ionizing radiation, ATM is activated by double-strand DNA breaks and phosphorylates the NH2 terminus of p53 at serine residue 18. We have previously generated mice bearing an amino acid substitution at this position (p53S18A) and documented a role for p53 phosphorylation in DNA damage–induced apoptosis. In this present study, we have crossed Eμmyc transgenic mice with our p53S18A mice to explore a role for ATM-p53 signaling in response to oncogene-induced tumorigenesis. Similar to DNA damage induced by ionizing radiation, expression of c-Myc in pre–B cells induces p53 serine18 phosphorylation and Puma expression to promote apoptosis. Eμmyc transgenic mice develop B-cell lymphoma more rapidly when heterozygous or homozygous for p53S18A alleles. However, Eμmyc-induced tumorigenesis in p53S18A mice is slower than that observed in Eμmyc mice deficient for either p53 or ATM, indicating that both p53-induced apoptosis and p53-induced growth arrest contribute to the suppression of B-cell lymphoma formation in Eμmyc mice. These findings further reveal that oncogene expression and DNA damage activate the same ATM-p53 signaling cascade in vivo to regulate apoptosis and tumorigenesis. Mol Cancer Res; 8(2); 216–22