PROM1 and ependymoma: Indeed, evidence for the existence of CD133-expressing cancer stem cell populations (clonogenic expansion in vitro and tumor-initiating capacity in vivo) has been provided in numerous tumor types including leukemia [28], prostate cancer [29], colon cancer [30], lung cancer [31], hepatocellular carcinoma [32], ependymoma [33], melanoma [34], ovarian cancer [35], medulloblastoma, and glioblastoma [2, 36].