Defective DSB occurrence and junction repair. (A) DSB occurrence in the Sμ regions of DNA agarose plugs from 20,000 B cells (CD19+) or non–B cells (CD19−) were treated or not with T4 polymerase. After ligation with the linker, ligation-mediated PCR products were transferred and Southern blotting performed using a 32P-labeled Sμ probe. DSBs were detected in the Sμ regions of control (n = 4) but not of P1 (n = 1) B cells. Amplification of Pinx1 was used as a control of DNA integrity and input. (B) Abnormal pattern of Sμ–Sα1 junctions in patients. Representation of the length of the switch junctions is shown (controls, n = 38; P1, n = 29; P2, n = 16; and P3, n = 15). Statistically significant differences are indicated (*, P < 0.05; and ***, P < 0.001 using the χ2 test).
