In agreement with this hypothesis we have found that tumors that developed from cells expressing the most potent inhibitors of tumor development, sema3A, sema3D, sema3E and sema3F, contained significantly reduced concentrations of blood vessels while sema3G which did not inhibit tumor development significantly also failed to reduce the density of blood vessels in tumors (Fig. 3). The gene discussed is SEMA3F; the disease is neoplasm.