Expression of BCL6, MUM1, and CD138 in monoclonal B-cell PTLDs. (A) Diffuse large B-cell lymphoma with centroblastic morphology displaying the BCL6⁺/MUM1^-/CD138^- phenotypic pattern. Medium-sized tumor cells show a nuclear staining pattern with the anti-BCL6 MoAb. Residual plasma cells show nuclear brown staining for anti-MUM1 antibody and cytoplasmic red staining for anti-CD138 MoAb (arrows). (B) Diffuse large B-cell lymphoma with centroblastic morphology displaying the BCL-6^-/MUM1⁺/CD138^- phenotypic pattern. Most neoplastic cells show strong nuclear immunoreactivity with the anti-MUM1 antibody. No BCL6 expression is detectable in the neoplastic population; residual plasma cells show cytoplasmic staining for anti-CD138 MoAb. Paraffin-embedded tissue sections, immunoperoxidase method (BCL6, MUM1), APAAP method (CD138), hematoxylin counterstain. Original magnification, × 250.