BaxBak doubly deficient T-cell progenitors fail to differentiate in an OP9-DL1 culture system. Mx-Cre⁺BaxBak DKO and MxCre-Bax F/⁺Bak^−/− (control) mice were treated with Poly (I; C) to induce Cre expression. Hematopoietic progenitor cells were isolated from BM and induced to differentiate over a period of 4 weeks by culture on OP9-DL1cells in the presence of interleukin-7 and fms-like tyrosine kinase receptor-3. (A) Nonadherant hematopoietic cells were collected on 14, 21, and 28 days and CD4 and CD8 expression in thymocyte progenitors were analyzed by flow cytometry. (B) CD4-CD8-Mx-Cre⁺BaxBak DKO and control cells were analyzed for CD25 and CD44 expression. (C) Hematopoietic progenitor cells collected from the OP9-DL1 cultures were stained with annexin V to identify and quantify apoptotic cells by flow cytometry. Percentages of annexin V and propidium iodide positive and negative quadrants are shown below each dot plot. Data are representative of 3 independent experiments.