PROM1 and colorectal cancer: Cell surface CD133 has been shown to be a stem cell marker in normal and neoplastic cells (Mizrak et al, 2008), including human colorectal cancer stem cells (O'Brien et al, 2007; Ricci-Vitiani et al, 2007).Cell surface expression of CD133 was increased on Colony 29 cells grown on type I collagen, suggesting that type I collagen maintains the stem cell phenotype.