CXCR4 and leukemia: BMSCs can protect leukemia cells against chemotherapy drugs through various cytokines or growth factors (e.g., PDGF, VEGF, EGF, SCF, etc.)or cell-cell surface contact (e.g., SDF1/CXCR4, VLA4/VCAM1, CD44/E-selectin, etc.)(Manabe et al., 1994; Wilson and Trumpp, 2006; Weisberg et al., 2008; Zeng et al., 2009; Sison and Brown, 2011).