Similar to a previously published report [14], the Mdm2+/+Arf+/+p53−/− mice primarily developed one tumor type with only 2 mice (11%) having two primary tumors, whereas the Mdm2+/−Arf+/+p53−/− mice and all other genotypes developed two tumor types with a frequency of 18–29%. Here, MDM2 is linked to neoplasm.