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. This evidence concerns the gene EFNB2 and neoplasm.