The tumour suppressor gene p53 is mutated in ~50% of human cancers. The p53 protein can be activated by a broad range of stress stimuli, most notably DNA damage, hypoxia and oncogene activation. p53 can in turn activate multiple cellular effector processes, including apoptosis, cell cycle arrest, DNA repair and cellular senescence. It has been postulated that p53 suppresses tumorigenesis mainly, if not exclusively, by inducing apoptosis. This appears, however, unlikely, given that puma-/-and even puma-/-noxa-/- double knock-out mice are not tumour prone despite the fact that Puma and Noxa account for all of p53’s apoptotic function. Remarkably, even mice that lack puma and p21even those lacking puma, noxa and p21, the major inducers of apoptosis and G1/S phase cell cycle arrest, respectively, are not tumour-prone. Hence, we hypothesize that p53 must suppress tumorigenesis by additional, less-characterized pathways. To identify critical tumour suppressor mechanisms we are using in vivo RNAi library screens using transduced and transplanted hematopietic stem cells from wildtype, puma-/-p21-/-and puma-/-noxa-/-p21-/- mice. Initial results yielded several p53 regulated candidate genes with tumour suppressive potential. Importantly, preliminary confirmatory experiments validated some of these candidates and thus confirmed their tumour suppressive function.