EPHB4 and neoplasm: We therefore hypothesized that, similar to the repulsive effects observed between the EphB4- and ephrin-B2-expressing endothelial cells during vasculogenesis, an analogous molecular mechanism acts as a “natural barrier” of extravasation in the spinal endothelium, enabling the repulsion of circulating tumor cells (CTCs) and thus hindering extravasation and metastasis formation.