To assess whether CXCR4 mediates direct interactions between GBM and endothelial cells we turned to an in vitro co-culture model similar to that used by others [8], [16], in which primary human brain microvascular endothelial cells (HBMECs) and either U87 cells, or primary GBM cell isolates were cultured together in extracellular matrix (Matrigel). The gene discussed is CXCR4; the disease is glioblastoma.