p53 is a tumor suppressor gene mutated in >50% of human cancers, while p53 deficiency in mice results in cancers and accelerated mortality. Thymic T cell lymphoma is the most common malignancy in p53-deficient mice, making it difficult to study the role of p53 in other malignancies. To overcome this limitation, we attempted to generate mice with a reversible p53 knockout (p53(rev/rev)) by inserting a floxed transcriptional stop into the first exon of p53, anticipating that this would allow tissue-specific Cre-mediated expression of p53. Contrary to expectations, functional p53 protein was expressed in the thymus and multiple other tissues of p53(rev/rev) mice in the absence of Cre, whereas B cells expressed p53 protein only in the presence of B cell-specific CD19-Cre. In the absence of Cre, 76% of p53(rev/rev) mice developed splenic marginal zone B cell lymphomas, indicating sensitivity of this B cell subset to transformation caused by p53 deficiency. 5'-RACE identified p53 mRNA transcribed from a novel start site utilized in thymocytes but not normal B cells or B cell lymphomas from p53(rev/rev) mice. The p53(rev/rev) mouse thus demonstrates an effect of p53 deficiency in development of splenic marginal zone lymphomas and provides a model for study of p53-deficient human B cell lymphomas.