Expression of STAT3 and BCL6 are mutually exclusive in normal GC cells. IHC and IF staining of normal tonsil GC for STAT3 and lineage- and differentiation-associated markers. The color for each marker is depicted by the label beside each panel. (A) A low-power image of BCL6 (red) and STAT3 (green) staining; note the STAT3⁺ cell localizing to the apical light zone. The high-power insets on the right are from an enlarged apical light zone area showing a rare cell (arrow) with nuclear BCL6 and cytoplasmic STAT3, whereas the other STAT3⁺ cells have no BCL6. (B) Triple staining for BCL6, PRDM1/Blimp-1, and PY-STAT3 shows mutually exclusive nuclear PY-STAT3 and BCL6, as well as PY-STAT3 and PRDM1. Rare double STAT3⁺PRDM1⁺ cells are noted (arrow). The rectangle is selected, and the image is split into a single-color panel below. (C) Double staining for STAT3 and PY-STAT3 shows that these 2 signals are always colocalized (shades of yellow color). (D) Double staining for PY-STAT3 and CD3, a pan-T marker, shows that there is no overlap between these 2 markers. (E) Double staining shows that all STAT3⁺cells are positive for CD20, a pan-B marker. (F) Double staining for PY-STAT3 and CD138/Syndecan, a late-stage plasma cell marker, shows that these 2 markers are mutually exclusive. Scale bar, 4 μm in the low-power image of panel A, 1 μm in other panels.