PECAM1 and neoplasm: Finally, the density of vascular structures, revealed by labelling blood vessels with an antibody to the endothelial marker CD31, significantly increased by 1.5-fold in MDA-MB-231-β1 tumours, compared to control tumours, and VEGF secretion in vitro was significantly higher in MDA-MB-231-β1 cells than control MDA-MB-231 cells (p < 0.01; Figs. 3g−3i).