Developmental failure at the transition step from DN3a to DN3b in ATM^−/− thymocytes. (A) Representative dot plots showing CD4 and CD8 profiles from ATM^+/+ and ATM^−/− mice. (B) Absolute numbers of total thymocytes in ATM^+/+ and ATM^−/− mice. (C) Relative percentage of lineage-negative cells. (D) Representative dot plots are gated on double-negative (DN) cells from ATM^+/+ and ATM^−/− mice, when DN3 cells reached at 1 × 10⁴ cells. Red dots indicate DN3b cells that are back-gated from CD27⁺icTCR-β⁺ fraction. (E) Bar graph represents DN1, DN2, DN3, and DN4 thymocyte percentages. (F) Dot plots are gated on DN3 cells (CD25 low, indicated in red in the top panels). DN3a cells are shown as CD27^low intracellular (ic) TCR-β⁻. DN3b cells are CD27^highicTCR-β⁺ and are indicated as red dots in red squares. (G) Bar graph represents icTCR-β–positive DN3b thymocyte percentages in the DN3 fraction. (A-G) The data were obtained from 12 ATM^+/+ mice and 11 ATM^−/− mice at 4 to 8 weeks of age. (H) Dot plot data gated on DN3 cells from ATM^+/+ and ATM^−/− mice. The icTCR-γδ–positive cells, which are indicated as red dots in the black squares, are CD27⁺icTCR-γδ⁺. (I) Percentages of icTCR-γδ–positive DN3b thymocytes in the DN3 fraction. (H-I) Data were obtained from 6 ATM^+/+ mice and 5 ATM^−/− mice. Data are representative of at least 3 independent experiments (mean ± SE). *P < .05, ***P < .001.