Significantly, CD133-expressing cells in glioblastoma and colorectal cancers include, but are apparently not limited to, the small subpopulation of tumour cells termed cancer stem cells (CSCs) which mediate tumour initiation and metastasis (Singh et al, 2004; O'Brien et al, 2007; Ricci-Vitiani et al, 2007). This evidence concerns the gene PROM1 and colorectal cancer.