High frequency of expression of the proliferation marker Ki 67 in YB-1+ MM cells. (A) Immunohistochemical analysis of Ki 67 protein expression in situ in YB-1−, mature MM cells (left), or YB-1+, immature MM cells (center) or anaplastic MM cells (right). *Negatively stained megakaryocyte. (B) Immunohistochemical analysis staining either of YB-1 (red; left) or Ki 67 protein (brown; center) or both (red and brown; right) in YB-1+, immature MM cells. Images were acquired as described in “Immunohistochemical analysis.” (C) Ki 67+ MM cells were counted to determine the growth fraction. Expression of Ki 67 is significantly more frequent in YB-1+, morphologically immature, or anaplastic PCs than in YB-1−, morphologically mature PCs. Also shown is the median of the percentage of Ki 67 staining in MM samples.