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Abstract: Signaling pathways such as the pre-TCR and Wnt pathways regulate α/β T cell differentiation in thymus. Mice lacking an essential component of the pre-TCR exhibit arrest at the (CD4−CD8−) (CD44−CD25+) stage (DN3) of thymocyte development, and introduction of p53 deficiency into those mice abrogates this arrest, resulting in transition to the (CD4+CD8+) double-positive (DP) stage. This paper examines the effect of inactivation of p53 on thymocyte development in Bcl11b−/− mice that exhibit arrest at the DN3 or (CD4−CD8+) immature single-positive (ISP) stage. No DP thymocytes were detected in thymocytes of adoptive transfer experiments in scid mice that were derived from p53−/−Bcl11b−/− precursors but ISP thymocytes increased in the proportion and in the cell number approximately three times higher than those from Bcl11b−/− precursors. Consistently, the level of apoptosis decreased to the level of wild-type precursors. These results suggest that inactivation of p53 is sufficient for DN3 thymocytes to differentiate into the ISP, but not to DP, stage of thymocyte development in Bcl11b−/− mice. This provides evidence for a novel p53-mediated checkpoint that regulates the transition from the DN3 to ISP stage of thymocyte development. [Copyright &y& Elsevier]