WT1 is expressed in CD138⁺ plasma cells in the BM of MM patients. The expression of WT1 in MM cells was longitudinally assessed over the course of disease in MM patients before and after alloTCD-HSCT and DLI. Paraffin-fixed BM biopsies from MM patients were double stained with monoclonal antibodies to CD138 (MI15; DAB, brown) and WT1 (6F-H2; nFu, red). Immunohistochemical analysis of WT1 expression was performed and biopsies were graded negative, focal, +, ++, +++, or ++++ based on the percentage of CD138⁺ PC staining positive for WT1. Representative biopsy stains are shown. (A) ++++ > 75% of CD138⁺ PC stain positive for WT1; UPN5. (B) ++ > 25%-30% of CD138⁺ cells are WT1⁺; UPN10. (C) Focal < 5% of CD138⁺ cells are WT1⁺; UPN10. (D) Negative, no CD138⁺ are positive for WT1; UPN10 (20×).