but this notion was challenged by studies from other groups. CD133 was found widely distributed in many epithelial tissues, and CD133 expression does not correlate with the ability of colon tumors to metastasize, as 40% human CRCs that metastasized to the liver are CD133 negative (11). CD133 expression can also be modulated by oxygen levels (17). Therefore, CD133 as CSC marker need to be re-evaluated and additional CSC surface markers maybe involved in maintaining CSC properties (18). To evaluate whether CD44, CD133 or combination of both can represent CSCs of CRC, we analyzed their protein expression levels in several CRC lines by western blotting.