CXCR4 and leukemia: A20 leukemia cells express the CXCR4 chemokine receptor that guides them toward a chemokine gradient of CXCL12 (stromal cell-derived factor-1, SDF-1) actively produced by the biliary epithelium and bone marrow stromal cells (42, 43), which may account for the preferential metastatic behavior of A20 leukemia cells for these tissues.