However, as will be detailed below, CD133 may not necessarily be the most accurate marker for tumor cells that display the functional characteristics that have come to be associated with TSC, and recently, several groups have suggested that CD15 (stage specific embryonic antigen 1 or SSEA-1), which is expressed on neural progenitor and stem cells, may be a better marker than CD133 of tumor-initiating cells in MB, glioma, and ependymoma [32-35]. This evidence concerns the gene PROM1 and central nervous system cancer.