B-Cell Maturation Antigen (CD269) as a Predictor for Response to Treatment in Multiple Myeloma Patients. Rania Mohammad Abdullah1* , Ahmed Embaby2 , Gehad Hamed 1 ABSTRACT Background: More research is going on markers for multiple myeloma (MM) to improve treatment. National Comprehensive Cancer Network International Prognostic Index (NCCN-IPI) is used for risk evaluation and treatment tailoring in MM patients. Relatively, few studies assess role of B-Cell Maturation Antigen (BCMA) in Egyptian MM. The aim of this study was to assess BCMA expression among MM patients and evaluate its prognostic significance. Methods: Thirty-six newly diagnosed MM patients were enrolled in the study. BCMA was assessed by multicolor flow Cytometry (MFC) using BD FACS CANTO II flow cytometer. Three months following chemotherapy, patients’ bone marrow (BM) were examined for evaluation of their remission status. Results: BCMA (CD269) was expressed on malignant plasma cells of all MM cases of the studied group. There was a statistically significant difference in CD269 expression among MM groups according to the international staging system, being highest in stage ІІ, also there was a significant difference in the expression of CD269 among MM patients by serum immunofixation being higher in the IgG type than IgA type. A highly statistically significant lower expression for CD269 was observed in MM patients who achieved remission. A significant positive correlation between CD269 Mean Fluorescence Intensity (MFI), and serum Lactate dehydrogenase (LDH) was detected. Moreover, a highly significant positive correlations between CD269 MFI, serum β2- microglobulin and CD56 MFI were observed. Conclusion: High BCMA expression was associated with low remission rate, also it was associated with high CD56 MFI, serum LDH and serum β2- microglobulin Keywords: MM; BCMA; MFC; Chemotherapy; Remission