CXCR4 and B-cell chronic lymphocytic leukemia: The chemokine CXCL12 is constitutively secreted by bone marrow stroma cells in CLL and binds CXCR4 on the surface of CLL cells to direct chemotaxis, support tumor survival, and activate various signaling pathways, including STAT3 by phosphorylation of its serine 727 residue (6, 9).