Novel inhibitor of histone acetyltransferase repressor (NIR) is a transcriptional corepressor with inhibitor of histone acetyltransferase activity and is a potent suppressor of p53. Although NIR deficiency in mice leads to early embryonic lethality, lymphoid-restricted deletion resulted in the absence of double-positive CD4(+)CD8(+) thymocytes, whereas bone-marrow-derived B cells were arrested at the B220(+)CD19(-) pro-B-cell stage. V(D)J recombination was preserved in NIR-deficient DN3 double-negative thymocytes, suggesting that NIR does not affect p53 function in response to physiologic DNA breaks. Nevertheless, the combined deficiency of NIR and p53 provided rescue of DN3L double-negative thymocytes and their further differentiation to double- and single-positive thymocytes, whereas B cells in the marrow further developed to the B220(+)CD19(+) pro-B-cell stage. Our results show that NIR cooperate with p53 to impose checkpoint for the generation of mature B and T lymphocytes.