Thymic stromal cells of ATM^−/− mice support DN3a to DN3b transition normally. Bone marrow cells from ATM^+/+ or ATM^−/− mice (Ly5.2) were transferred into lethally irradiated ATM^+/+ mice (Ly5.1). The thymi were harvested from the reconstituted recipient ATM^+/+ (Ly5.1) mice at 4 weeks after BMT. (A-C) Total thymocytes, TCR-β–positive cells, and TCR-γδ–positive cells were calculated on CD45.2-positive cells. For each group, more than 3 mice were analyzed. (D) Flow cytometric data from ATM^+/+ thymocytes (Ly5.1) reconstituted with ATM^+/+ or ATM^−/− BM progenitors (Ly5.2). Cells were gated on CD45.2-positive and lineage-negative fractions and analyzed for CD25 and CD117 marker expression. Histograms show icTCR-β positivity in the DN3 phase. DN3b cells as defined by icTCR-β positivity were back-gated to the DN3 fraction and are indicated as red dots. Data are representative of 3 independent experiments. (E) Percentages of DN1, DN2, DN3, and DN4 cells in ATM^+/+ (Ly5.1) recipient mice that received either ATM^+/+ or ATM^−/− BM progenitors (Ly5.2). Cells were gated on CD45.2 and analyzed. (F) Percentages of icTCR-β–positive DN3b thymocytes in the DN3 fraction in ATM^+/+ (Ly5.1) recipient mice transplanted and gated as in panel E are shown. Data are mean ± SE. **P < .01, ***P < .001.