Expression of the plasma cell–associated marker MUM1/IRF4 in neoplastic lymphoid cells.

(A) Bone marrow biopsy specimen showing infiltration by multiple myeloma. Monoclonal MUM1/IRF4 is present in the nuclei and the perinuclear area in neoplastic plasma cells. Residual hemopoietic elements are unlabeled. The inset shows higher magnification-view of the same field. (B) Strong labeling of tumor cells in classic HD (APAAP technique in paraffin sections). Original magnification A, × 600; A inset, × 1000; B, × 1000.