CD24 and breast cancer: In addition to their resistant phenotype, Al-Hajj et al. (2003) showed that CSCs are more tumorigenic than non-stem cancer cells, such that breast cancer cells expressing CD44 (CD44+) but not CD24 (CD24−/low) are more tumorigenic since as few as 100 cells with this phenotype are able to form tumors in mice while millions of cells missing this phenotype are not.