The ubiquitin interaction motif-containing protein RAP80 was recently found to play a key role in DNA damage response (DDR) signaling by facilitating the translocation of several DDR mediators, including BRCA1, to ionizing irradiation (IR)-induced foci. In this study, we examine the effect of the loss of RAP80 on genomic stability and the susceptibility to cancer development in RAP80 null (RAP80-/-) mice. RAP80-/- mice are viable and did not exhibit any apparent developmental defects. Mouse embryonic fibroblasts (MEF) derived from RAP80-/- mice underwent premature senescence compared with wild-type (WT) MEFs, were more sensitive to IR, and exhibited a higher level of spontaneous and IR-induced genomic instability. RAP80-/- thymocytes were more sensitive to IR-induced cell death than WT thymocytes. RAP80-/- mice were more susceptible to spontaneous lymphoma development and the development of 7,12-dimethylbenz(a)anthracene-induced mammary gland tumors. Moreover, the loss of RAP80 accelerated tumor formation in both p53-/- and p53+/- mice. Our data indicate that RAP80-deficiency promotes genomic instability and causes an increase in cancer risk consistent with the concept that RAP80 exhibits a tumor suppressor function. ©2012 AACR.