Kaposi's sarcoma-associated herpesvirus (KSHV) encodes a cyclin D homolog, K cyclin, that is thought to promote viral oncogenesis. However, expression of K cyclin in cultured cells not only triggers cell cycle progression but also engages the p53 tumor suppressor pathway, which probably restricts the oncogenic potential of K cyclin. Therefore, to assess the tumorigenic properties of K cyclin in vivo, we transgenically targeted expression of K cyclin to the B and T lymphocyte compartments via the E micro promoter/enhancer. Around 17% of E micro -K cyclin animals develop lymphoma by 9 months of age, and all such lymphomas exhibit loss of p53. A critical role of p53 in suppressing K cyclin-induced lymphomagenesis was confirmed by the greatly accelerated onset of B and T lymphomagenesis in all E micro -K cyclin/p53(-/-) mice. However, absence of p53 did not appear to accelerate K cyclin-induced lymphomagenesis by averting apoptosis: E micro -K cyclin/p53(-/-) end-stage lymphomas contained abundant apoptotic cells, and transgenic E micro -K cyclin/p53(-/-) lymphocytes in vitro were not measurably protected from DNA damage-induced apoptosis compared with E micro -K cyclin/p53(wt) cells. Notably, whereas aneuploidy was frequently evident in pre-lymphomatous tissues, end-stage E micro -K cyclin/p53(-/-) tumors showed a near-diploid DNA content with no aberrant centrosome numbers. Nonetheless, such tumor cells did harbor more restricted genomic alterations, such as single-copy chromosome losses or gains or high-level amplifications. Together, our data support a model in which K cyclin-induced genome instability arises early in the pre-tumorigenic lymphocyte population and that loss of p53 licenses subsequent expansion of tumorigenic clones.