FIGURE 24-13 Conditional mouse models of cancer. In the inactivating system (a), an exon of interest is flanked by two loxP or FRT sites as shown. Expression of the Cre or FLP recombinase leads to homologous recombination between the two loxP and FRT sites, respectively. This recombination leads to excision of the exon, rendering the gene nonfunctional. In the activating system (b), an additional exon with a stop codon is introduced into the gene of interest, making the gene nonfunctional. This exon is flanked by loxP or FRT sites. When Cre or FLP recombinase is induced, the stop codon–containing exon is recombined out, and the gene of interest is expressed.