Double immunostaining results showed that within microtumors without blood perfusion (<400 μm, Fig. 3) the Ki67+ cells in microtumors were proximal to CD31+ endothelial cells (Fig. 5A), and more ECs and Ki67+ cells were detected in bigger microtumors (Fig. 5B), suggesting ECs may play an important function during initial tumor growth prior to blood perfusion. This evidence concerns the gene PECAM1 and neoplasm.