TIMP3 and neoplasm: Cluster 1 ECs (referred to as peripheral EC type I [Pe1] because it originates mainly from the brain tissue surrounding the tumor mass) displayed a quiescent endothelial marker profile, as expected of normal brain ECs, and were characterized by a high expression level of genes implicated in vascular integrity (KLF2, TIMP3) and BBB function (SLC2A1, SLCO1A2) (Figure 3A, Supplemental Table 4, and Supplemental Figure 5).