MDM2 and neoplasm: To test whether Mdm2 haploinsufficiency would still inhibit tumor development in the absence of p53, we evaluated tumorigenesis in Arf−/− mice that also lacked both alleles of p53. Mdm2+/−Arf−/−p53−/− mice and littermate Mdm2+/+Arf−/−p53−/− controls had analogous rates of tumor development resulting in similar Kaplan-Meier survival curves (Fig. 1C).