YB-1 expression is lacking in situ in normal PCs, in PCs of patients with MGUS, and in MM cells with a mature PC morphology. (A-C) Immunohistochemical analyses of YB-1 protein expression in situ, (A) in normal PCs of the BM, (B) in premalignant PCs from a patient with MGUS, or (C) malignant PCs with a mature phenotype from 2 patients with MM. PC morphology was highlighted by staining according to Giemsa. A normal PC (marked by an arrow in panel A), PCs from patients with MGUS (marked by arrows in panel B), and morphologically mature-appearing MM cells (C) do not express YB-1, which seems to be, however, sometimes expressed in granulopoietic cells (*). Images were acquired as described in “Immunohistochemical analysis.”