YB-1 is strongly expressed in situ in MM cells with immature and anaplastic morphology. (A,B) Immunohistochemical analysis of YB-1 protein expression in situ, or (A) in morphologically immature or (B) anaplastic PCs. Biopsies shown in panel A were obtained from the BM of 2 patients with MM; biopsies in panel B were obtained from different extramedullary sites of 2 patients with MM. Strong, diffuse predominantly cytoplasmatic expression of YB-1 is observed in morphologically immature MM cells (A) and in morphologically anaplastic MM cells (B). Each patient panel includes a Giemsa stain to highlight PC morphology. Arrows in panel A indicate a prominent nucleolus, which is a typical feature of immaturity. Arrows in panel B indicate large, multinucleated, bizarrely formed PCs, which are characteristic of an anaplastic phenotype. (C) Immunofluorescence images of primary MM samples stained for YB-1 (Cy3; red), IRF4 (Cy5; green) and nuclei (DAPI; blue). YB-1 is in situ detectable in the cytoplasm and nuclei of MM cells and is organized in dot-like structures (white arrows) inside the nucleus. Nuclear YB-1 is colocalized with transcriptionally active euchromatin (weak DAPI staining). Staining of IRF4 was performed to mark PCs. The insets show details of the picture in a 2-fold magnification. Images in panels A and B were acquired as described in “Immunohistochemical analysis,” and in panel C as in “Confocal microscopy.”