p53 is a tumor suppressor protein encoded by the Tp53 gene. Its role in cell cycle regulation and stabilization for preventing genome mutation is observable among a wide variety of multicellular organisms, including humans, rodents, frogs and fish. Heterozygous rats deficient in p53 protein are prone to spontaneous tumors, making them valuable for in vivo screening of carcinogenicity, as well as studying chemopreventive and therapeutic treatment.