PECAM1 and glioblastoma: In a prior study, we did not find a significant correlation between microvascular proliferation and cfDNA concentration in GBM.5 In a patient-derived orthotopic xenograft GBM model, Mair et al.20 also did not find a significant relationship between microvessel density (measured by CD31 expression as an endothelial cell marker) and ctDNA concentration.