PECAM1 and neoplasm: Platelets have been localized to well-established GSC-related niches, specifically necrotic and hypoxic regions of the tumor, in prior studies, but expression proximal to GSC has not been described.6,7,15–17 Immunofluorescence (IF) staining of GBM patient specimens demonstrated SOX2-positive tumor cells (GSCs marker) and CD61-positive platelets in and around the vasculature, identified by CD31 (Figure 1a, Supplementary Figure S1).