More detailed in vivo studies including limiting dilution studies will be required to determine the percentage of cells within the expanded tumour sphere populations that exhibits CSC properties as performed previously for freshly isolated CD133+-enriched populations (O’Brien et al, 2007) or CD133+ short-term passaged tumour spheres (Ricci-Vitiani et al, 2007; Vermeulen et al, 2008). This evidence concerns the gene PROM1 and neoplasm.