The essential roles of the CXCL12/CXCR4 axis in niche positioning and cell cycle status of leukemia stem cells have been highlighted by the specific deletion of CXCL12 from BM MSCs, suggesting the differential use of CXCL12-niches by CXCR4+ malignant cells (13). The gene discussed is CXCR4; the disease is leukemia.