CXCR4 and glioblastoma: In our view, the decrease in CXCL12 secretion by GBM cells under such low oxygen conditions, as well as the increased expression of CXCR4 (its receptor) in response to hypoxia [56], may play a functional role in forcing tumor cells to migrate along the CXCL12 gradient, directing them to perivascular areas where oxygen conditions are improved.