Nonequivalent TCR-β recombination and an increase in DNA DSBs in ATM^−/− thymocytes. (A) Rearrangement status of TCR Vβ1-Jβ1, Vβ4-Jβ1, Vβ8-Jβ1,Vβ12-Jβ1, Vβ15-Jβ1, and Vβ16-Jβ1 in the DN2, DN3a, DN3b, and DP stages was analyzed by PCR. (B) γH2AX-positive cell percentages in the DN1 to DP phases by flow cytometry. (C) Percentage of cells in S phase determined by flow cytometric analysis for PI and EdU incorporation at the DN2 to DP stages in ATM^+/+ (n = 3)and ATM^−/− thymocytes (n = 4). (D) Apoptotic cell percentages determined by flow cytometric analysis for annexin V positivity in the DN3a and DN3b stages. Data are mean ± SE. *P < .05.