MDM2 and neoplasm: Enhanced tumor suppression and normal aging is also observed in Mdm2puro/Δ7–12 transgenic mice, which have one hypomorphic and one null allele of Mdm2, express ∼30% of the wild-type level of Mdm2, and have constitutively high TP53 activity (Mendrysa et al., 2006, 2003).