Mdm2 heterozygosity significantly inhibited tumor development in the absence of Arf, and in contrast to Myc oncogene-driven cancer, this delay in tumorigenesis could not be rescued with the presence of one allele of Arf. Notably, Mdm2 haploinsufficieny blocked the accelerated tumor development in Arf deficient mice caused by p53 heterozygosity. The gene discussed is TP53; the disease is cancer.