PECAM1 and glioblastoma: GBM microcirculation was assessed by histological quantification of expression of pan-endothelial markers (CD31, CD34, and von Willebrand factor – vWF) as well as the marker of newly-formed microvessels (CD105 - endoglin) together with the expression of the major proangiogenic vascular endothelial growth factor (VEGF) and the brain-specific activator of angiogenesis in the hypoxic condition – hypoxia inducible factor 2 alpha (HIF2α).