MDM2 and neoplasm: Firstly, to determine the requirement of p53 in mediating the delay in tumorigenesis in Mdm2+/−Arf−/− mice and the effects of altering the gene dosage of p53, we generated Mdm2+/−Arf−/− mice and littermate Mdm2+/+Arf−/− controls lacking one allele of p53 and monitored them for tumor development.