In addition, although Mdm2 haploinsufficiency inhibits tumor development in Arf−/−p53+/− mice, these mice still have a mean survival that is significantly shorter than Mdm2+/+Arf+/−p53+/− (Fig. 3, Table 1), indicating biallelic loss of Arf is dominant in this situation. The gene discussed is MDM2; the disease is neoplasm.