Studies taking advantage of mouse strains expressing ‘switchable’ p53 genes have uncovered a crucial role of p53 reconstitution in regression of already established tumors—lymphomas, soft tissue sarcomas, and hepatocellular carcinomas (Martins et al., 2006; Ventura et al., 2007; Xue et al., 2007). This evidence concerns the gene TP53 and lymphoma.