MDM2 and cancer: Consistent with this mechanism, transgenic mice with an increase in TP53 copy number (García-Cao et al., 2002) or a hypomorphic Mdm2 allele have elevated basal TP53 activity and are resistant to tumor formation (Mendrysa et al., 2006), indicating that shifting the TP53-MDM2 equilibrium away from TP53 degradation can directly promote cancer resistance.