CXCR4 and neoplasm: SDF-1 receptor CXCR4 is heterogeneously expressed by a subpopulation of MM cells,18 and its presence on the cell surface of primary MM cells highly correlates with BTK expression.16 This suggests that distinct intraclonal subpopulations of MM cells are involved in tumor-cell adhesion, proliferation and metastasis to new BM niches.