Thymopoiesis in IL-7Rα-/- mice is blocked at the CD4-CD8- double negative stage with failures in T cell receptor (TCR) γand β rearrangement. Early studies suggest that unsuccessful TCRβ rearrangement in mice defective in DNA damage repairs, incurring DNA double-strand breaks, results in p53 activation. However, it remains unknown whether p53 plays a role in the thymopoietic defect in IL-7Rα-/- mice. In this study, we demonstrated that p53 is highly activated in the thymocytes of IL-7Rα-/- mice. Considering the importance of p53 activity during TCR rearrangement and the observation of incomplete rescue of thymopoiesis in IL-7Rα-/- mice by modulating activities of Bcl-2 family members, we hypothesize that p53 activation may also contribute to the impaired thymopoiesis in IL-7Rα-/- mice and that temporal suppression of p53 activity by the IL-7Rα signaling facilitates thymopoiesis. With the generated IL-7Rα-/-p53-/- (DKO) mice, we found that p53 inactivation in IL-7Rα-/- background resulted in a 30-fold increase in thymic cellularity with increased TCRβ? cells. Furthermore, DKO mice developed thymic lymphoma at a higher incidence and died significantly earlier than p53-/- mice. Our study is the first to illustrate a functional interplay between IL-7Rα signaling and p53 pathways in facilitating TCRβ+ T cell development during thymopoiesis and lymphomagenesis.