PROM1 and glioblastoma: When biopsies of glioblastomas were investigated, CD133+ cells proved to be a minor population of cells varying from 1 to 3% of total tumor cell population; however, clinical studies suggested that the percentage of CD133+ cells [25, 26] or rate of tumorsphere formation [27, 28] predicted overall survival of the patient (although contrary evidence also exists [29]).