CLL cells that divided and up-regulated AID protein exhibited more dsDNA breaks. (A) Confocal photomicrographs comparing CLL PBMCs stimulated in the CD40 + IL-4 system with unstimulated cells cocultured only with CD32-transfected fibroblasts. Original magnification was 630×. (B) Quantitative colocalization of CFSE intensity (x axis) and anti-pH2A.X staining (y-axis) on CD23⁺ cells derived from stimulated cultures. The shaded area (gray) represents the range of pH2A.X intensity derived from unstimulated cells, all of which had a CFSE intensity of at least 256 pixels; numbers denote the quantity of cells present in each of the 4 quadrants. ***P < .0001 by Fisher exact test. (C) Graph showing the change in AID mean fluorescence intensity (MFI) identified in CD5⁺CD19⁺ cells of the same 3 samples in panel B as determined by flow cytometry.