Engineered p53 mutant mice are valuable tools for delineating p53 functions in tumor suppression and cancer therapy. Here, we have introduced the R178E mutation into the Trp53 gene of mice to specifically ablate the cooperative nature of p53 DNA binding. Trp53 mice show no detectable target gene regulation and, at first sight, are largely indistinguishable from Trp53 mice. Surprisingly, stabilization of p53 in Mdm2 mice nevertheless triggers extensive apoptosis, indicative of residual wild-type activities. Although this apoptotic activity suffices to trigger lethality of Trp53 ;Mdm2 embryos, it proves insufficient for suppression of spontaneous and oncogene-driven tumorigenesis. Trp53 mice develop tumors indistinguishably from Trp53 mice and tumors retain and even stabilize the p53 protein, further attesting to the lack of significant tumor suppressor activity. However, Trp53 tumors exhibit remarkably better chemotherapy responses than Trp53 ones, resulting in enhanced eradication of p53-mutated tumor cells. Together, this provides genetic proof-of-principle evidence that a p53 mutant can be highly tumorigenic and yet retain apoptotic activity which provides a survival benefit in the context of cancer therapy.