Early pro-B cells from FLT3/ITD mice accumulate DSBs and are defective for DSB repair. (A-B) γH2AX foci are increased in early B-cell progenitors from FLT3/ITD mice compared with wild-type littermate controls. Images were acquired at room temperature using a Nikon TE 2000-E microscope system (Nikon) with a Nikon Plan APO VC 100×/1.40 oil objective (original magnification ×1000) and Nikon EZ-C1 Version 3.5 software. Flow cytometric analysis shows increased phosphorylation levels of H2AX in early pro-B cells from FLT3/ITD mice (C), with an increased mean fluorescence for cells in G₀/G₁ phase (D), compared with their wild-type controls. Early pro-B cells from FLT3/ITD mice demonstrate decreased NHEJ efficiency (E) and increased errors (F) after repair of DSBs. Repair efficiency is assessed as the number of total bacterial colonies obtained from the transfection of 5 × 10⁶ sorted early pro-B cells. Misrepair rate is calculated as the fraction of white in total (blue and white) colonies. Data are expressed as mean ± SEM (error bars). Early pro-B cells from FLT3/ITD mice demonstrate an increased frequency of larger deletions (G) and the use of longer microhomologous sequences (H) for the repair of DNA. Data are representative of 3 independent experiments.