PECAM1 and glioblastoma: Because ISH staining for the prototypical endothelial marker CD31 (PECAM1) is not available in the Ivy GAP, analysis of glioblastoma vasculature considered ESM1, MECOM, and ENPEP; genes known to be enriched in endothelial cells [21,22] (Fig. 1B-1D and S1B-S1D).