CXCL12 and acute lymphoblastic leukemia: To further verify the role of SDF-1/CXCR4 axis in the interaction between TGF-β conditioned MSCs and ALL cells, CXCR4 was overexpressed in Nalm-6/RS4;11 cells through lentiviral transfection, the successful up-regulation of CXCR4 was verified through RT-PCR and Western blot methods (Supplementary Figures S3A,B), and the effect of TGF-β conditioned MSCs on the migration and invasion of leukemia cells was further tested.