CXCL12 and glioblastoma: CXCL12/CXCR4 axis, at least in part, contributes to GBM pericyte formation inducing migration of CXCR4-expressing CSCs toward the perivascular niche, where ECs secrete CXCL12 (Ehtesham et al., 2009; Folkins et al., 2009) and TGF-β drives differentiation into mature pericytes (Cheng et al., 2013).