Tissues from Mdm2I438K/I438K; p53KO/KO and Mdm2I438K/WT; p53KO/KO mice at 5 mo of age exhibited similar organ morphology and histological analysis revealed that both sets of mice had developed small to medium-sized thymic lymphomas independently of MDM2 status by this time (Supplemental Fig. S2B), consistent with the normal disease trajectory of p53KO/KO mice (Donehower et al. 1995). This evidence concerns the gene MDM2 and thymus lymphoma.