AID is expressed in Pax5⁺ GC and extrafollicular B cells. (A) AID transfectants (ii, purple) but not mock-transfected cell lines are stained by the EK2-5G9 antibody. Original magnification × 40. (B) Low-power image (original magnification × 10) of AID (purple) and panB cell Pax5 (red) in tonsil. The epithelial surface is on the right. Two rectangles indicate areas magnified in panel C. (Ci) AID stains Pax5⁺ GC and extrafollicular large blasts. (Cii) Subepithelial Pax5⁺ B cells are AID⁺. Original magnification × 40. (D) AID⁺ GC cells are BCL6⁺ (red), while extrafollicular AID⁺ are BCL6^–. Original magnification × 20. (E) Low-power image (original magnification × 10) of AID (purple) and MUM1 (red). The star marks plasma cell collection in the septa. AID⁺ GC on the right is MUM1 negative. (F) High-power image of a MUM1^–AID⁺ GC (star) and MUM1^– extrafollicular blasts. Original magnification × 40. (G) AID⁺ centroblasts (red) are PRDM1^– (purple). These latter are found in the AID^– light zone (star). Original magnification × 40. Scale bar: 1 μmat × 40, 4 μmat × 10, and 10 μmat × 4.