In an in vitro transwell-based invasion assay we revealed reduced migration of B-ALL cells, if they had been cocultured on MMP-9-deficient compared with wild type MSC, to the chemoattractant stromal-derived factor (SDF)-1α (P = 0.05, Figs. 3g and S5, A–B) or the B-cell chemoattractant C-X-C motif chemokine ligand 13 (CXCL13) (P = 0.013, Fig. S6A), for which B-ALL cells express the corresponding C-X-C motif chemokine receptor 5 (CXCR5) (Fig. S6B). The gene discussed is CXCL12; the disease is precursor B-cell acute lymphoblastic leukemia.