MUM1 expression in myeloma cell line and normal lymphoid tissues.

(A) IM9 human myeloma cell line. Microgranular positivity for the MUM1 protein is observed in the nucleus (stronger) and in the cytoplasm (weaker) of tumor cells. Arrowheads indicate negative nucleoli (APAAP; × 1000). (B) MUM1 protein expression in the nucleus (stronger) and in the cytoplasm (weaker) of plasma cells in a lymph node involved by a plasmacellular variant of Castleman disease (paraffin section; APAAP; × 800). (C) Paraffin section from normal tonsil double stained for cytokeratins (tonsil epithelium labeled in brown) and MUM1 (plasma cells labeled in blue) (APAAP; × 250). (D) All plasma cells in the tonsil epithelium double stain for intracytoplasmic light chains (brown) and nuclear MUM1 (blue) (× 1000). (E) Isolated or small clusters of MUM1⁺ cells (arrowheads) are present within GC (APAAP; × 250). (F) At higher magnification, some of the MUM1⁺ cells in the GC show a markedly irregular (often multilobated) nucleus (arrows). Arrowhead points to a MUM1⁻ macrophage (APAAP; × 1000). (A-F) Immunostaining with MUM1p monoclonal antibody; hematoxylin counterstain.