Inactivation of the p53 gene in the germline of mice by gene targeting has provided researchers with a model similar in many respects to the analogous human inherited cancer predisposition Li-Fraumeni syndrome. The viability of p53 null mice has allowed unexpected opportunities to study the role of p53 in many different in-vivo and in-vitro contexts. Null (p53-/-) mice have an average time to tumor development of 4.5 months, while half of the heterozygous (p53+/-) mice develop tumors by 18 months. The p53-deficient mice have been particularly valuable in examining the effects of p53 loss on tumor progression. In addition, the mice hold significant promise as tools to assess carcinogens, teratogens, chemopreventative agents, and cancer therapeutic regimens.