PROM1 and glioblastoma: However, conflicting findings have called into question the utilization of this marker in brain tumours: there is a high variability of CD133 expression in GBM (from 1% to 60%); and GBM stem cells not expressing CD133, which are able to self-renew and regenerate tumours in the xenotransplantation assay, have been identified [33–35].