SDC1 and plasma cell myeloma: Interestingly, evaluating intracellular expression of CXCR4 in myeloma PCs from BM biopsy specimens, we found higher intracellular CXCR4 levels in CD138+ cells of patients who failed to respond to BTZ compared with patients who responded, indicating that CXCR4 may influence response to chemotherapy in MM patients.