Proliferative fraction of freshly purified CD138⁺ cells according to disease stage. Freshly isolated MM cells were stained with APC-conjugated anti-CD138, followed by fixation/permeabilization before intracellular staining with FITC-conjugated anti-Ki67 and DNA-staining with PI. Analysis was on CD138⁺ cells, and the proliferative fraction in S/G₂M was gated as shown in panel A, which illustrates representative patients at different disease stages. EMD indicates extramedullary disease. (B) Cumulative data from 143 samples. Data are mean ± SEM for each group: SMM, 4; de novo MM, 31; plateau, 8; relapse, 91; PCL/PE, 9. SMM indicates smouldering MM; PCL/PE, plasma cell leukemia and myelomatous pleural effusion (includes 8 cases with PCL with data from BM CD138⁺ cells in 5, peripheral blood in 3, and 1 pleural effusion sample). Purified MM cells were used in all cases except for the postautograft cases where mononuclear cells were used.