CXCL12 and acute lymphoblastic leukemia: Taken together, these data indicate that the SDF-1/CXCR4 axis stimulates not only migration but also proliferation of ALL leukemic cells in vivo and in vitro, and implied the importance of targeting the extramedullary microenvironment to prevent recurrence from emerging from minimal residual disease in the extramedullary microenvironment in ALL patients.