NF1 and glioblastoma: 3) Mesenchymal GBM is the most invasive subtype and is characterized by the overexpression of angiogenesis markers (e.g., vascular endothelial growth factor {VEGF}, PECAM1), the loss of phosphatase and tensin homolog (PTEN) and neurofibromin 1 (NF1), and the activation of PI3K/AKT signaling, which are correlated with a poor prognosis.19