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. The gene discussed is CD44; the disease is breast cancer.