TP53 and neoplasm: Since Mdm2 heterozygosity greatly delays tumor development in Arf-null mice that have both alleles of p53, the difference in mean survival between p53+/+ and the p53+/− Arf-null Mdm2+/−mice was much larger than their Mdm2 wild-type counterparts and quite significant (p<0.0001, log-rank test; Fig. 2B, Table 1).