Mice lacking one or two copies of the p53 gene haveprovided invaluable insight into the process of tumorigenesis.The importance of apoptosis in suppression of tumorigenesisin vivo became evident from analysis of these mice.Moreover, the timing and kinds of tumors that develop inthese mice are altered by the presence of additional inheritedmutations, by strain differences, and by food intake. Developmentalabnormalities are also visible in mice with loss ofp53 and with overexpression of p53 suggesting that p53levels are critical for normal cellular processes. While mice donot necessarily recapitulate all the tumor types found ininherited cancers, they offer the unique opportunity to decipherthe critical pathways in tumorigenesis. These findingscan then be applied to humans.