TP53 and neoplasm: TP53 activation is regulated by the MDM2 ubiquitin ligase and in response to oncogenic stress, the p19Arf tumor suppressor sequesters MDM2, thereby preventing it from targeting p53 for degradation permitting the transactivation of p53-dependent genes to inhibit cell cycle progression among other functions (Stott et al., 1998; Weber et al., 2000; Sherr, 2006).