PBL of the oral cavity type in individuals infected by HIV. PBL tumor cells express markers associated with plasma cell differentiation such as CD138, CD38, and MUM1/IRF4. Tumor cells are usually negative for CD20, CD45, and CD56 (data not shown). H&E stain shows that the lymphoproliferation consists of medium-sized tumor cells displaying a cohesive pattern. Tumor cells show round nuclei with multiple nucleoli or few prominent nucleoli. The cytoplasm is generally large and basophilic. Immunostaining with the plasma cell markers MUM1/IRF4 and CD138 is strong in PBL. The pattern of staining is nuclear for IRF4/MUM1 and cytoplasmic and membranous for CD138. MUM1 and CD138, immunohistochemistry, hematoxylin counterstain. Original magnification, ×400. Images were taken using a Nikon Eclipse 80i microscope with a Plan Fluor 40×/0.75 objective and Nikon digital sight DS-Fi1 camera equipped with control unit-DS-L2. Images were processed using Adobe Photoshop 6.