PECAM1 and acute lymphoblastic leukemia: Quantification of the area fraction occupied by CD31‐positive endothelial cells (Fig. 4B) showed that both VEGF‐expressing populations caused a significant expansion of vascular structures compared to controls (ALL 13 ± 5% and SPEC 9 ± 1% versus PBS 6 ± 1% and CD8 6 ± 1%).