Thus, it is conceivable that the presence or absence of CD133+ endothelium co-purified with CD133+ GSCs may be an important factor influencing the tumorigenicity of GSCs that have been derived directly from surgical specimens but not of GSCs isolated from more homogeneous cell populations such as established glioma cell lines, which are less likely to contain human endothelial cells. This evidence concerns the gene PROM1 and central nervous system cancer.