FLT1 and neoplasm: Lambrechts and co-workers classified tumor endothelial cells in different clusters based on the marker genes identified; Lymphatic endothelial cells (PDPN+, PROX1+), tumor-derived blood endothelial cells (FLT1+, IGFBP3+, and SPRY1+), malignant, and non-malignant endothelial cells (23) (Figure 1).