A number of different tumor cells (rat T9 glioma cells [12]–[14], human U251 glioma cells [15], [20], rat MADB106 breast cancer cells [weak immunogenic, 26] and mouse Hepa1-6 hepatocellular carcinoma cells [non–immunogenic, 17]) that express the novel membrane form of M-CSF lack the ability to form either subcutaneous or intracranial tumors in rodents. This evidence concerns the gene CSF1 and central nervous system cancer.