DNA-PKcs kinase activity promotes efficient signal join formation in ATM-deficient pro-B cells. (A) Southern blot analyses of the rearrangement status of pMX-DELSJ V(D)J recombination substrate in ATM−/− v-Abl–transformed pro-B cells with or without DNA-PKcs inhibitor (either NU7026, marked as 26, or NU7441, marked as 41). (B) Fidelity analysis of the chromosomal signal joints recovered from ATM−/− cells with pMX-DELSJ substrate in the presence or absence of DNA-PKcs inhibitor NU7441. (C) Southern blot analyses of the rearrangement status of pMX-DELSJ V(D)J recombination substrate in WT cells treated with ATM inhibitor, DNA-PKcs inhibitor, or both together. For both A and C, the DNA was digested with EcoRV and probed with the indicated C4 probe. The UR band is 5 kb, the SJ band is 4 kb, and the 3′SE band is 2.2 kb.