PECAM1 and glioblastoma: To determine if pp71-expressing cells are proximal to endothelial cells in primary tumor specimens, we performed double immunofluorescence on frozen GBM tissue samples and found that pp71 positive GBM cells were indeed located adjacent to and surrounding the CD31 positive endothelial cells, further supporting our hypothesis for the pp71-induced paracrine signaling promoting GBM angiogenesis (figure 4D).