CXCR4 and acute lymphoblastic leukemia: CXCR4 is commonly expressed on the surface of ALL and acute myeloid leukemia (AML) cells.[11] Similar to its role in hematopoietic stem cell (HSC) regulation, activation of CXCR4 by SDF-1 is critical for the migration and retention of leukemia cells within the bone marrow and may play a role in extramedullary spread.[12] Because the SDF-1/CXCR4 axis is important in attracting and retaining leukemia cells within the bone marrow microenvironment, CXCR4 inhibition could allow leukemia cells to be released from protective niches that contribute to chemotherapy resistance.