CXCL12 and glioblastoma: The small but reproducible effect of CXCR4 antagonism on GBM cell localization suggests that the CXCL12-CXCR4 pathway participates in localization of primary GBM cells to the peri-endothelial cell domain but does not account for all of the chemo-localization effect of endothelial cells on GBM cells.