TP53 and neoplasm: In addition to the constitutive germline-knock-in or knockout of p53, a Cre-LoxP system (Cre acts as a site-specific recombinase to catalyze recombination between specific LoxP sites, and thus depletes genes or gene fragments flanked by LoxP sites) has been applied to conditionally knock in or knock out p53 in mice and to address the role of p53 at different tumor developmental stages [54].