The plasma-cell proliferation index (PCPI). (A, patient no. 3; B, patient no. 4) Representative multiple myeloma bone marrow biopsy specimens double-stained with anti-CD138 and anti–Ki-67 antibodies. The majority of cells in these sections are plasma cells, which exhibit crimson membrane staining with anti-CD138 mAb. Green arrowheads indicate positive nuclear staining (dark brown pigment) for Ki-67. Yellow arrowheads denote light blue nuclei that do not stain for Ki-67. Original magnification × 50. (C) PCPI and real-time RT-PCR for Ki-67 mRNA were compared for patient (Pt) no. 3 and no. 4 from panels A and B, respectively. PCPI is expressed as percentage of Ki-67–positive plasma cells within the CD138⁺ population. Error bars show the standard error of the mean.