While it is consistently expressed among the many stages of B-cell development, the level of CD38 expression varies, and further increases in density during differentiation to plasma cells. in patients with relapsed and refractory MM may or may not express conventional markers due to altered biology. These findings suggest there may be gradually increasing diagnosis- and treatment-related challenges in relapsed MM in the future. Multiparametric flow cytometry (MFC) can distinguish normal from neoplastic plasma cells, provide prognostic information regarding progression risk in monoclonal gammopathy of undetermined significance (MGUS) GNF-7 [5] and smoldering multiple myeloma (SMM), and detect minimal residual disease in patients with MM post-therapy [6]. Plasma cells exhibit characteristically bright CD38 and CD138 expression, and are further defined by the expression GNF-7 of CD45, CD19, CD20, CD27, CD28, CD56 and CD117, amongst others [7,8], forming the basis for the European Myeloma Network consensus methodology [8] on plasma cell analysis by flow cytometry. We describe a novel case of CD38 unfavorable MM, its impact on the interpretation of flow cytometry analysis and its biological and clinical implications. In brief, our patient, a 71-year-old African American male, presented with refractory MM in August 2010 after originally being diagnosed >10 years ago and having several lines of therapy, including melphalan/prednisone, autologous stem cell transplant in 2001, and lenalidomide, cyclophosphamide, bortezomib, bendamustine and pomalidomide. Recently, he displayed evidence of progressive disease during his last course of therapy, with a rising monoclonal protein concentration and development of a 3 4 cm soft-tissue abdominal mass. As part of the clinical work-up, a bone marrow biopsy was performed and routinely processed for histopathological and immunohistochemical evaluation. Bone marrow aspirate and a fine needle aspirate (FNA) from the abdominal extramedullary mass were sent for flow cytometry analysis. Specimens were acquired GNF-7 using an eight-color multiparametric approach on a three-laser FACS Canto II (BD Biosciences, San Jose, CA) with DiVa 6.1.1 software and analyzed by FCS MULTI-CSF Express 3 software (DeNovo Software, Los Angeles, CA). Morphological evaluation of the bone marrow biopsy revealed a hypocellular marrow with a monoclonal population of CD138 (+), CD56 (+), CD117 (+) and kappa (+) but CD38 unfavorable plasma cells constituting 1015% of the total cell population [Figs. 1(IA)1(IG)]. Flow cytometric evaluation of the bone marrow aspirate showed that this plasma cells were CD138 (+), CD38 dim (+), CD19 (), CD20 (), CD27 dim (+), CD45 dim (+), CD56 bright (+), CD117 (+), with monoclonal intracytoplasmic kappa light chain expression [Figs. 1(IIE)1(IIL)]. In the abdominal extramedullary mass, plasma cells were CD138 (+), CD38 (), CD19 (), CD20 (), CD27 dim (+), CD45 (), CD56 (+), CD117 () [Figs. 1(IIA)1(IID)]. The CD38 () plasma cells were identified using CD138, CD56 and forward scatter. Corresponding cytological evaluation exhibited atypical plasmacytoid cells.Table Iillustrates the immunophenotypic differences of clonal plasma cells between bone marrow and extramedullary mass, highlighting the evolutionary immunomodulation in the neoplastic cells. To our knowledge, neoplastic plasma cells lacking CD38 expression are a novel finding. The intensity of CD38 expression by neoplastic plasma cells may often be slightly decreased as compared to normal plasma cells [7,8], but CD38 dim GNF-7 (+) expression is reportedly rare [9]. == Physique 1. == (I) (A) Paraffin sections of bone marrow biopsy showing hypocellular bone marrow with clusters of plasma cells (hematoxylin and eosin staining). Plasma cells are positive for CD138 (B), CD56 (C), CD117 (D), and show kappa light chain restriction (F). CD38 expression (E) is usually markedly dim to unfavorable. Plasma cells are unfavorable for lambda light chain (G) expression. (Original magnification 200.) (II) Multiparameter (eight-color) flow cytometric immunophenotyping of an abdominal mass fine needle aspirate (FNA) and bone marrow aspirate (FACSCanto II Flow Cytometer, BD Biosciences; FSC Express v3 analysis software, DeNovo Software). Plasma cells are highlighted in red, with background viable cells in gray, when present. Plasma cells of the FNA were CD138 (+), CD38 (), CD19 (), CD20 (), CD45 (), CD56 (+), CD27 dim (+) and CD117 () (A-D). Plasma cells of the bone marrow aspirate were CD138 (+), CD38 dim (+), CD19 (), CD20 (), CD27 dim (+), CD45 dim (+), CD56 bright (+), CD117 (+), with monoclonal intracytoplasmic kappa light chain expression (E-L). Isotype controls were utilized (I, J) for intracytoplasmic light chain staining. == Table I. == Immunophenotypic variation of malignant plasma cells from extramedullary mass and bone marrow. +, positive; , unfavorable. In the standard diagnostic setting, primary gating of.